To me, this seems fine when rare. So let's say I rent out a room once a month or something along those lines - no big deal. If I'm treating it like a real b-n-b, though, it seems like I'm engaged in some real commerce that is probably regulated. Presumably Airbnb behaves agnostically on this issue, allowing the responsibility to fall to the property owner?
I'm delighted for the specific homeowners being discussed here, but concerned about the general case.