Fan fiction can actually be a major problem for the author ...
For example, Terry Pratchett quite openly says on the fan forums he frequents, "NO fan fiction". It's not that he's against it per se, but he is (quite rightly) nervous that someone will post fan-fic, and then he'll use it because he doesn't remember reading it and thinks he made it up - or, more likely - that he'll use a similar plot and get accused of stealing the fan-fic story.
So the forum moderators and posters basically have a choice - ban fan-fic and welcome the author, or allow fan-fic and drive the author away.
Terry doesn't particular care which way a forum goes, but he does care that forums he frequents abide by his rules.