It's the same thing as with Lucas though. Harry Potter is bigger than its creator now. It's Frankenstein's monster. The fans, like with Star Wars, are the franchise now.
The same thing befalls lots of musical groups which is why they tend to call it quits instead of giving in to what their fans want from them. Metallica tried to do what it wanted and their fans revolted, so Metallica gave up and decided giving the fans what they want and taking their money made more sense.
Rowling can say and do whatever she wants to try to control the Harry Potter product, but it belongs to the fans now, and the fans spend the money and the fans get what they want. That's just how this stuff goes.