It is a matter of principle and courtesy. It is one of the main perks of this website and the way we can tell who to do business with.
If you have to ASK someone to reciprocate it is like fishing for a compliment. I just won't do business with them again.
I completely agree that's a common courtesy that's should be reciprocated without request, but sometimes folks get tied up in other things and forget. I give them the benefit of the doubt.