I've said this before in this forum, and probably will again...
The sad truth about fanfic writers is, like so many other things, it only takes one to ruin it. Anne McCaffrey initially allowed Pern fanfics, until (as I recall the story) something that was written in one of her novels had been portrayed in a fanfic. Rather than accepting that she'd simply had the same idea, the fanfic author claimed McCaffrey had copied her and looked into legal action. In the end, McCaffrey had no choice but to officially demand that no fanfics be done based on her work.
Here's one of the dangers. If you can write even a halfway decent story and decide to do one based on someone else's work, there's going to be some overlap. Often times an author foreshadows things well enough that it's easy enough to come up with at least some of what they're going to do! If people thought "cool, I figured out what they were going to write" then no harm, no foul. The problem is people who decide they've been robbed and start harassing the author.
I remember a case of a Harry Potter fanfic writer who rewrote the first book from Hermione's POV. Same story, same characters, different POV. At the top of the story, that was posted on her website, she listed that she held the copywrite on all characters and the story! Warner Brothers, who'd just paid a fortune for some of those rights, was somewhat perturbed.
The bottom line is this. Writing fanfics isn't bad per se. The problems occur based on what you do with the fanfics, especially in the Internet age. Sadly, enough fanfic authors have caused the original authors problems (and cost them money) that they have to respond.
Even George Lucas, who has no problem with Star Wars fanfics, has had to stand up at least once. About a year ago we had a discussion here about "Star Wars: Another Hope".