The fact that you share a common enemy does not make you allies.
Walk into a room with 10 strangers and smack each of them in the mouth on your way by.
All of a sudden 10 strangers who don't know each other and have nothing in common (other than the smack in the mouth that you just gave them) have a common goal.
Are ANY of them in the wrong for wanting to return your smack in the mouth?
It is an OOC restriction for game balance. Attempting to make sense of it by real-world analogy will fail.
It is exactly as real and logical as only being able to move one region per turn (even if it only takes 6 hours to get from region A->B->C), or only being able to recruit troops in the capital (even if the recruitment centers are out in the rural regions).
