Well, the way I see it is that by defeating the avatars, they are bound to protect you no matter what. Therefore, if you get aggro, they will attack the mob to protect.
Pets however, are subject to your will; you charm them, and force them to do your bidding against their will. They don't desire to help or protect you, and therefore, they will not protect you of their own volition.
Automatons, if they don't protect you(not sure how they work), are machines, and as such require user input to perform any action.
As for wyverns, I dont know. Maybe they are just lazy?
