"So that means Rowling has no control over her fantasy universe, even when it's just reflecting what she wrote?"

That is right, and it's by design, as long as you're talking about the concepts and ideas of her universe and not their specific expressions. But again, as I said, it's not clear where the line between idea and expression lies with particular parts of a work of fiction.

The free exchange of ideas is essential to the working of a free society. I can't imagine that much good would come of making the distribution of all ideas subject to the will of the people that came up with them.