One of the things is that these are very small infringements, in a way, and they're done by a lot of people. If one of these works, these mashups, starts circulating around the Internet, one of the things that could happen is that the cumulative effect of all these small infringements, or one infringement that goes viral on the Internet, in fact has a disastrous effect on a particular thing.
What if somebody writes a sequel to my novel? Maybe I think that's fine as fan fiction, but maybe it's not. Maybe they've picked up the clues I had in my earlier novel and have written the sequel I was going to write. Then it's copied on the Internet and people think I've copied my fan.