The most common situation seems to be one where it is legal for Airbnb, the business, to exist and do what it does, but where the majority of the people listing their places to rent on it are violating some law/regulation or clause of their lease by doing so.

Sort of like how running a user-generated content site is legal, but people who upload movies/albums/etc. to it in violation of copyright are on the wrong side of the law.