in most escorts profiles here they will receive but don't give I see, maybe its discretion but most will not do it as don't find licking a guys ass not that appealing really.
However I found that British and Irish girls are keen on doing it and in fact like it. I would say its depends too on what you look like, what kind of ass you have and is it well looked after, probably a combination of all these things, I don't think its anything to do with being selfish, more to do with what you like and turns you on