But what else could Warner Bros have done at a point where they were embarking on making a franchise based on a projected 7 book series of which only 3 books had been written/published? They either had to have involved Rowling in the process or they risked their films wildly diverging from the books and setting up all the fan sturm und drang that comes with that. Given the twists and turns of Rowling's narrative, there's no way the film makers could have anticipated where she was going with the story and we'd likely have ended up with an entirely different Harry Potter story.
And what's the point of that? It would have turned Harry Potter into a mess of GAME OF THRONES proportions.