Does Warner Bros own the copyright to the books now?
Just browsing trademark filings the agreement is WB has copyright on the movies and Rowling the books and everything based on them which gives WB the ability to go after conventions, think Disney and Snow White. Disney doesn't own the book but is so associated with the story they have carts Blanche to throw their weight around. Obviously that's my take on it and not gospel.
The truth is that those fans are eating into the recently opened Wizarding Worlds profits, they should go to the theme park and enjoy high priced copyright protected fun, not have a half priced more fun convention.
I really don't know. That's a very good question.