Dunno!
This is the crux of the problem. The definition of "commercial" has never been tested in the courts. As it currently stands, however, it is understood to mean that private residential dwellings may not be let out except to friends and family. Now we debate what "friends and family" are, and that is something that hasn't been tested in the courts either!
Any "advert", paid or free, which introduces a third party unknown to the owner, is not allowed unless the property it is advertising has, or is part of a property that has, a touristic licence. And now we debate whether a website is an advert or not.
Absolute minefield.