Inspiration from a book, or a tv show that uses a universal trope like vampires, or werewolves, or the undead, isn’t the same thing as taking literal text from a source and dropping it into a document that you then publish as your work. If the AI program was merely finding a trope, that would be one thing. But once it begins to incorporate things someone else wrote, well, that’s the slippery slope.
All of my fan fiction is online. Quite a few original short pieces are online as well. If an AI program were to scour the Internet, and decide to lift sentences or even paragraphs from one of my original stories, I’d be pretty damned peeved, I can promise you, and there would be a DMCA takedown notice going out forthwith. (AFF does actually have a DMCA statement in the archive’s Site Information, as any reputable site should.)