True enough, but I don't think it was only that. Producer/director/screenwriter influence and budget aside, I'm willing to bet that Rowling had a hand in making blatant changes to the script so that people who otherwise wouldn't buy the book, would.

Think about it: the majority of people going to these movies have read the books, yet a good number probably didn't. At the movie's end, the very first thing that the book-readers will talk about is what did or didn't really occur in canon. And normally it's a significant bit. It'd be enough to pique anyone's interests, and that's a whole slew of extra post-book release book sales. I witnessed this having worked at a book store all last year; we sold a ton of OotP copies after the movie dropped.