It's incredible to me that he would have no say. Not that he is the be all end all.
I am not sure if Harry Potter is the best example. You could be right, it could have just been a lucky matter of respect and fandom. What I read--and following the logic I don't think it can be false--is that the movie makers HAD to involve Rowling becase all of the books were not written/published at the time the movies started being made. She had to approve changes and cuts because she was the only person who knew how everything impacted the final outcome, and what could be adjusted and what could not, in that context.