Probably in the "zero[0] marginal cost to owner" sense. They already paid for them; that cost has been sunk. If they aren't using them (and while I don't have citations to back this is, I'm confident that the majority of privately owned bathrooms are sitting idle and unused at least half of the day) why not let someone else?

[0] Yes, okay, fine, the marginal cost is not truly, literally zero: there'll be a few cents for water, a little wear and tear on the equipment, a few squares of toilet paper, a couple squirts of soap, etc. However, most people do not track their budgets that closely anyway, so it would be lost in the noise.

On the website it says that if you only want to host, you don't need to pay the membership fee.

So the fee is really only if you want to be a guest. But then you need to host to earn credits to be a guest.