There's a difference between "technically, we reserve the right to refuse service to anyone at any time with no warning" and deliberately misleading people about how their rules will be applied. Bait and switch
is illegal, regardless of what the TOS says. If they've enforced their rules in a particular way in the past, and made statements about how they intend to enforce it in the future, and do something else--that's against the law.
If they didn't technically break the law, it's still psychotically bad business practice to do so.
I don't believe fanfic is intellectual property theft--and it's a matter the courts have been unclear on. At a minimum, fan fiction falls into a middle ground between a film of a book and a "musical composition inspired by a novel."
Certainly if the fic in question is parody, it falls under fair use. (It is my belief that all HP slash is parody, as it mocks the "children's novels can't have erotic content" concept.)
Since affect on sales of the original, and commercial gain of the derivative work, are both part of the consideration in copyright infringement cases, fanfic has a heavy push in the "legal" direction--I don't believe any original works have been financially damaged by fanfic; quite the opposite. (I'm aware that's not the only consideration. But two of the four considerations are "money.")
Every one of the couple-hundred slash fans on my f'list bought at least one copy of Harry Potter 7. Some Americans bought one for every member of their family, plus the British edition.
However, I don't know any Anita Blake fanficcers. The author doesn't "allow" fanfic. (Whether that would hold up in court is unknown; currently, no large fanarchive site is willing to host content that the author actively objects to.) Don't know any Anne Rice fanficcers; same issue.
I do know that none of those couple-hundred slash fans buy their books. The general attitude is, "if you don't want people to play in the world you created--if you want your universe to be a diorama that sits untouched on the shelf--we'll play somewhere else."
Fanfic is no different from a review or essay about the original work... it's another form of exploration of the ideas in, and commentary on the meaning of, the original.