[MereInterest]

> I thought there was nothing stopping you from creating derivatives of copyrighted works as long as you don't try to exploit them commercially.

Nope, creating derivative works is something that is explicitly part of copyright, whether or not it is for commercial gain. Fanfiction or fan recreations are only legal with the consent of the author. Typically, authors will turn a blind eye to it, since it increases the longevity of the original works. It is only a civil matter, and so only the copyright holder could bring a suit against fanfiction authors.