Well, it’s just a metonymy. All sorts of horrible things are totally legal, and all sorts of inoffensive things are not, so I certainly wouldn’t be ashamed to be called an illegal.
But I’m not going to use it that way any more, since I’ve been asked not to.
That’s pretty much what happened to me, at the Red Lion in Lacock Village! The staff was having none of it, but they were very good natured about the whole thing.