My opinion is that I don't consider authorized tie-in novels fanfic for a couple of reasons. Tie-in novels are contracted and paid for by the publisher, which gives them legal legitimacy. Also, the kinds of worlds with tie-in novels tend not to belong to one person's exclusive creative domain. Comic books, television shows and game settings, even if thought up by one person originally, usually have a team of writers working on it as is so contracting out to more writers isn't as unusual. You're not going to find tie-in novels with Dresden, Alera, etc. because the novels ARE the product being sold. In all cases, though, I don't consider it fanfic if it's put out by the publishers of the original source material with their stamp of approval on it.
Fanfic is fun for the fan, but the problem lies in fans taking material that belongs to someone else, and making it their own instead. When does it cease to be the author's baby, and start to be anyone's? That's the tricky question, and to avoid it I absolutely understand authors having to protect their authority over their work. That's why any fanfic'ish stuff I do, is kept entirely private between me and if applicable who I'm gaming with if it's being used as a setting. Plus, I never use someone else's characters. Maybe it's the gamer in me, but I feel that a person's characters are their own unique sliver of their own personality and creativity. That's all theirs, and I can't imagine taking over someone else's voice.