Let's say that, by some miracle, my novel becomes a bestseller. Let's say, in fact, that it becomes an instant sensation, and that readers all over the world are so fascinated with the characters and the world that they start producing their own art based on my book: drawings, and short stories, and games, and so forth. (Disregard for a moment that there's no Tolkien-level world-building in my current novel; this is all just a thought experiment.) At this point, my book has become something of a modern myth, a story that everyone is familiar with at some level, to the point where my characters and situations are used in everyday conversation.
At that point, is the fictional world in the book still mine?
I'm sure that it's mine legally. But is it really mine, all mine? Hasn't it become part of our common culture? I created it, yes, but should I be allowed to dictate how, precisely, it can and cannot be used? Should I be allowed to impose a blockade around it, so that no other creative works can be derived from it? What if someone else is able to produce something based on my book that has its own value, its own merit? If so many people are fascinated by the world that I've created, do I have the right to stop someone else from telling a different story in the same world that's just as enjoyable as mine?
I've only recently come across this story from 2008, where a Harry Potter fan decided to produce a lexicon of words and terms from the Harry Potter universe, and J. K. Rowling shut him down. The linked article quotes her as saying that "this book constitutes the wholesale theft of 17 years of my hard work."
Well. I would suggest that a collection of words and definitions is hardly wholesale theft; if the would-be author had simply photocopied all seven books cover to cover, I might agree with this statement; otherwise it smells a bit of hyperbole.
Ms. Rowling is also quoted as saying "I went to court to uphold the right of authors everywhere to protect their own original work. The proposed book took an enormous amount of my work and added virtually no original commentary of its own." If that's the case, would she be okay if I produced a totally original Harry Potter book, with a new story? That would be quite a bit of original commentary, wouldn't it? Surely she would be okay with that?
But of course she wouldn't. I would get hit with a cease and desist letter so hard that you'd hear the impact on Mars. Ms. Rowling didn't take action here because she was concerned that someone was copying her work. Her motivation is clear from the words she used: specifically, "protect" and "theft".
This is about money. Someone else was going to make money off of the Potterverse. Was Ms. Rowling planning to write her own Potteresque dictionary? Certainly not; she's fled from the realm of fantasy and whimsy into the more respectable neighborhood of literary fiction. Was this proposed dictionary going to render Ms. Rowling penniless? No, I expect that her accountants would never even feel the bump. But if she lets this one labor of love onto the market, it opens the door for further "theft". If she doesn't maintain her fence around Hogwarts, then everyone will be inside.
And that would be bad because...?
The world is full of creative people with great ideas. If they're inspired and excited by a certain idea, should we prevent them from sharing their creations with the world, just because someone else got there first?
(P.S.: For a much more eloquent presentation of this discussion, check out Pirate Cinema by Cory Doctorow.)