According to Henry Jenkins, director of comparative media studies at MIT, humans have always reinterpreted the stories they hear. If anything, what's recent is the inflexibility of media corporations in responding to fans' creations. Not that many fan filmmakers would turn down an offer to join the big studios. While technology enables fan filmmakers to achieve near Hollywood effects, it has not delivered the key to good filmmaking. The acting and screenwriting in most fan films are not exactly of Oscar-winning quality, and other production values tend to betray the films' amateur origins.
Parodies seem to come up better than the serious films.
I guess if the film's not taking itself too seriously, it's easier for the audience to do the same. But, again, the effects aren't half-bad. That's partly due to the immense amount of sharing on the Internet among fan filmmakers when it comes to special effects and other production techniques. As for the legal issues, fan films appear to fall within a gray area between fan fiction and parody, both of which are legally protected under U.S. law. Within the fan-film scene, de facto rules keep most fan filmmakers from selling their work.