Learn English – “A better future tomorrow” vs “a better tomorrow”

As used in your example, tea is like a meal (such as lunch or dinner) where you only have tea and whatever goes with it.
So the form is the same as lunch. You would not say the lunch, if used in place of tea, in your example.
I don't believe it is an idiom.

It's no different than saying:

My aunt is coming to dinner tomorrow.

So, I think the confusion comes from the fact that there are two things that 'should be' can mean.

The first, as Peter explains, is something that is anticipated, or assumed. By saying

He should be there

We mean that we expect or assume he is or will be there. The time frame generally depends on context, it could be future or present.

Present:

Frank should be in a meeting right now. (It is my assumption that Frank is in that meeting)

Future:

Frank should be in that meeting tomorrow. (It is my assumption that Frank will attend meeting)

The other way that should be can be used is to state your opinion that you think ought to happen.

So for your example:

There should be a law against it.

Means that 'I believe that they need to create a law to prevent that'.

Unfortunately, you can even use both versions of 'should be' in the same sentence, but the context changes the meaning. For example:

The party tomorrow? I should be there.

I can't make the party tomorrow. Billy has his first game. I should be there.

Both examples use the same sentence, 'I should be there', but they have different meanings. The first means 'My current plan/assumption is that I will attend that meeting tomorrow.' The second sentence means 'I really ought to be there to support him.'