First, let's clear up a misconception.
Armor Penetration gives you a constant %damage bonus, regardless of how much armor the enemy has see here.
It doesn't matter if your enemy has 300 armor or 15 -- penetrating 10 of that will have the same effect on total DPS increase.
That said, let's take a look at why Armor Penetration is preferred, even with its linearity.
Attack Speed is easy to acquire via items.
Armor Penetration is not.
You can more than match the effects of attack speed runes with a single dagger - 420 gold.
The cheapest item with Armor Penetration is the Brutalizer at 1337 total gold. By this metric alone, Armor penetration runes are worth approximately three times that of attack speed runes.
It is easier to use items to stack Attack Speed over Armor Penetration -- but to get the highest DPS, you want to stack both.
Given that the amount of Armor Penetration available via items is strictly limited, if one's goal is to achieve maximal values for both Armor Penetration and Attack Speed, runes for Armor Penetration are much better than that of Attack Speed.
I find that item to be not that good for Olaf because it punishes you for building HP. It scales off HP missing on percent, not total, so the more health you have, the harder it is to get its passive in the max.
For Tryndamere, it is a very good item because you should not build HP on Trynd, so he can make use of that item very well.
Best Answer
I can confirm that his AS bonus is capped at 80%. I bought 40% worth of CDR and had 0.92 AS; after buying another Forbidden Idol, my AS stayed at 0.92.