In my experience every druid of the appropriate level has such a companion, but I have only GMed for one evil druid in Pathfinder (I have GMed many more evil druids in 3.5 but they don't face the no-evil-treants problem) and I suspect most evil pathfinder druids wouldn't have pulled off a permanent companion like that.
As you have noted, the treant is in no way compelled to serve you, which is a feature of the best druid minion spells. Like wizards, druids can call upon powerful creatures to serve them for free. Like clerics, druids can call upon powerful creatures to serve them with their full capacities and through their own free will. These spells keep the druid the ultimate 'summoner', in the sense that druids (and Nature in general) has the tightest knit community of allies. Fluff-wise this has been present in D&D since pretty much the beginning of the class and is integral to the D&D idea of druidic society and nature beings in general as a unified front formed of diverse and frequently opposed beings working together as needed to serve what they all hold to be the most worthy of service and devotion: nature. An evil druids with a good treant companion can still call upon them in times of need (nature's need, not their own) and be confident that the treant will assist them. The only issue is that, as neutral good beings, treants are likely to attack certain kinds of evil druids in most circumstances and generally not be good company for them. This is true in my experience in both 3.5 and Pathfinder.
You should also note that while in Pathfinder treants are 'NG outsiders' in 3.5 they were mearly usually neutral good. Evil druids could totally summon evil treants (and chaotic druids chaotic ones, etc.) if they want to. In practice though, most druids don't care enough about their own non-neutral alignment components to actually do that.
In response to the 'protector/guardian' line:
1) This is fluff. In 3.X fluff often contradicts the mechanical effects of an ability. Fluff should really not be treated as rules-text.
2) As a druid, you are a protector and guardian of nature. You typically work with and not over many, many other creatures to accomplish your goals. The animal companion class feature is prime example of this. You do not 'own' your animal companion. Your animal companion works with you of its own free will and, indeed, you have to make handle animal checks to make it do things it doesn't want to. Are you more powerful than it? Yes. But that doesn't make it your slave. It's really hard to have slaves as a druid. Animal slavery is almost always going to count as not revering nature, though there are certainly some ways around that. The highest-powered druid summons don't tend to get you creatures that are magically compelled to obey you in any way at all. Instead, they get you powerful allies that work with you because they also serve nature and, as a high level druid, you are nature (well, a part of it). cf. the effects of the spell World Wave and the 3.5 PhB class description.
3) What, exactly, is the treant guarding? You? The spell doesn't say. I'm much more inclined to rule that the spell animates a Tree/Treant as a protector/guardian of nature than of the animating druid. Of course, the distinction is largely irrelevant to most games, since the druid's and created treant's goals should align so much and the servants of nature are generally willing to lend other servants of nature a hand, unless (and sometimes even while) they are currently attempting to effect violence upon them.
Second part of the question:
Druid villains are pretty rare. I've yet to run into a druid that has a companion provided by this spell in published Pathfinder material that I've run, due to most of the druids encountered being below level 11. I know that Ashes at Dawn has a Vampire druid, Merrick Sais, who is certainly capable of casting the spell and who holds a position as a guardian fixture that would make a liveoak an extremely valuable companion. I don't know why she doesn't have one, but she doesn't. The other published druid villain I've actually run-- a treant himself-- doesn't really have enough detail given in his description to determine if he uses the spell. The worldwound supplement (where Carrock is printed) also contains an example Siabrae who might be able to cast the spell, but she similarly lacks description. These are, I believe, the only level 11+ published druids.
The tally, thus, is 1 definite no and two maybes.
Rangers Don't Get Foci
All classes that use a focus have a "Spellcasting Focus" subheading to their Spellcasting (or Warlock) feature:
- Wizard (arcane focus)
- Warlock (arcane focus)
- Sorcerer (arcane focus)
- Paladin (holy symbol)
- Druid (druidic focus)
- Cleric (holy symbol)
- Bard (musical instrument)
The wizards feature for instance reads:
SPELLCASTING FOCUS
You can use an arcane focus (found in chapter 5) as a
spellcasting focus for your wizard spells.
PHB 114
With the exception of the Ranger and Eldritch Knight, which specifically don't mention foci as part of the spell casting feature. Who have no such text.
It Is Intentional
Jeremy Crawford clarified in a tweet that the omission of focus for the ranger class is intentional. He was asked:
[D]o rangers use spellcasting foci, and/or do they need to buy component pouches at 2nd level?
And replied:
The ranger doesn't have a spellcasting focus. The trusty component pouch will do the job.
https://twitter.com/JeremyECrawford/status/687417277231267844?ref_src=twsrc%5Etfw
This makes sense as most rangers would likely have a bow. All bows, even the hand cross bow (even with Crossbow Expert), requires a free hand to load. So holding a focus would mess with the traditional ranger with bow. You keep a free hand for spells, pouch and loading firing arrows.
Mechanically
Using a bow, you have the weapon in one hand and the other on string. After you fire, your hand is free again. When you cast you pull out the components and put them back, or they are consumed, and your hand is free again. Works great with a bow. However, the arcane and druidic focus require object iterations, and you either have to drop it or spend the book keeping at the start and end of each turn.
Yes, a holy symbol could be worn and keep the hands free, likely why they did that for paladins, but they chose to go a different direction with the ranger.
Thematically
Ranger forage for food and materials, they are self reliant. It isn't hard to see them restocking their component pouch as they move through the wild area.
What about the Mistletoe?
it might be impossible to find mistletoe in the desert when a material component is needed.
The PHB says this about the component pouch:
Component Pouch. A component pouch is a small, watertight leather belt pouch that has compartments to hold all the material components and other special items you need to cast your spells, except for those components that have a specific cost (as indicated in a spell's description).
PHB 151
The pouch has all the components your spells require, and Material Components aren't consumed unless the spell says they are:
If a spell states that a material component is consumed by the spell, the caster must provide this component for each casting of the spell.
PHB 203
So, you have mistletoe once, and you have it forever -- unless your DM rules that lose it or it goes bad, etc. Then you have to buy more mistletoe or buy a new pouch. Such a thing, however, isn't in the rule.
Unearthed Arcana / Tasha's Cauldron of Everything
A recent released play test (UA), and page 57 of Tasha's Cauldron of Everything (optional rule) provides the following to the Ranger class:
Spellcasting Focus
2nd-level ranger feature (enhances Spellcasting)
You can use a druidic focus as a spellcasting focus for your ranger spells. See chapter 5, “Equipment,” of the Player’s Handbook for a list of things that count as druidic focuses.
Best Answer
Use the limitations of Travel via Plants to your advantage.
Secrecy is the first and best line of defense.
The rules for Transport via Plants suggest that you can only select a specific destination plant if it is familiar to you. Otherwise, you must specify desired distance and direction and the spell will transport you to the nearest plant of the same type as the origin plant to the location you specify.
So the invading druid would need one of two pieces of information. First they would need to be familiar with a specific plant inside the target grove. Failing that, they would have to have enough of an idea where the target grove is that they could specify distance and direction and jump to the nearest similar plant. With this in mind, the best way to keep other druids out is to prevent them from becoming familiar with the plants there or prevent them from knowing the location. So: operational security; keep a tight hold on information.
Populate your grove entirely with plants that are tiny in size or smaller.
Transport vis Plants states that "You can enter any normal plant (equal to your size or larger)" (emphasis added). If all the plants in your grove are tiny or smaller in size, then no Small or Medium (or larger) druids can transport there. A tiny druid could still pull it off, but this guards against the main character races if they don't use magical size reduction.
Populate your grove entirely with plant creatures instead of normal plants.
This rather extreme option takes advantage of the fact that "You can't use [Transport via Plants] to travel through plant creatures."
If possible, populate your grove only with unique or extremely rare plants.
If you can have a grove with unique plants that don't exist elsewhere, or a type of plant that is prohibitively difficult to find, you can preclude another druid from getting there with Transport via Plants since they need a plant of the same type to travel.