The purpose of copyrights is to ensure that it is profitable enough for an author to make the effort to produce a work. I believe the initial worry was that people would reprint and sell the work without paying the author. But all of this derivative stuff has gone too far, and prevents other artists from producing useful work.

What would happen if the law allowed this sort of "copying"? Rowling would still make enough for the effort to be worth it, and the defendants would make a pittance off of their guide. But what about the poor authors whose works did not sell as well? Nobody would copy them. Copyright law is out of control and inhibiting the art it is supposed to protect. It's time to change that.