It all depends. A "reader's companion" can be a compilation of scholarly and/or fannish essays on a writer's work (clearly not a violation of copyright -- Bethke recently wrote yet another piece for a series of books from a publisher, whose name I forget, that seems to specialize in doing not terribly but definitely arguably scholarly pieces on various famous stuff that they couldn't get the rights to play with if they had to buy or ask) or nothing more than a bunch of cut-and-paste's from the writer's work (clearly a violation), or something in between (which is what the lawyer's get paid for, if it gets to the lawyers).

But it's not just copyright; there's other IP issues, like trademark law involved, too.

My own take -- as just a guy who knows a bit about the issues and who took a quick look at a couple of JKR-related websites today; IANAL -- is that a lot of the fannish online stuff (particularly slash, which isn't the issue here) exists at the sufferance of the writers/copyright owner, and that JKR gets to decide if and/or when it comes down.