I think it has more to do with expectations than the money per se. AirBnB is first and foremost a business relationship, and it's probably common that neither person wants any more out of it. If I hit it off with the host I might recommend them to friends, but actually pursuing a friendship... probably not. Whereas getting to know the hosts is part of the appeal of couch surfing.

As another example, clubs and classes are a great place to make friends because you already have a common interest. You can become friends with the instructor/leader no matter what the monetary arrangements are. Whether there are dues or class fees, if the instructor gets paid a token or reasonable amount. Of course, the more of a profession it is for the instructor, the more likely they are to be spread thin across classes and students. But again, that's not the money per se.