Interesting thing about fan fic is that most companies are unofficially OK with it (as long as it doesn't get totally outrageous), but if it becomes provable that they are aware of the existance of a particular body of fan fic (such as if the author of said fan fic writes a letter to someone official in the company about it), the owner of the copyright must take the fan fic writer to court because to do otherwise is legally tantamount to just walking away from the copyright on the material that the fan is writing about. All this has to do with a requirement for a copyright holder to "make a good faith effort to protect copyright" in order to maintain it.
Fan Fic