There is not anything that says this, that's just the way I believe it should be, for many reasons. Put simply, I don't think anyone has the right or the ability to keep complete control over something as large and important as Harry Potter.
That's fair enough. That's essentially what I'm arguing too, but in reverse. I guess that's just a matter of opinion.
What if Linus didn't want anyone else to improve on Linux?
Then it would have died out in favor of some open source thing. But it was Linus's choice. Say a writer could gain a massive popularity surge by open-sourcing her world and characters. She'd have an edge over Rowling because Rowling won't do that. But either way, it's not the law allowing her to get ahead, it's the market. If Rowling is ignored for some more open world, then the market has spoken. If, however, her ideas stay big despite her retaining outright control, then there's no incentive to take them from her, and my opinion is that it's morally wrong to.