Over at John August's blog, he brought in this recent LA Times article, There follows a great discussion on the strings and arrows of a novelist's demands and furies after a novel was adapted into an underperforming film.
A lot of comments brewing, good arguments on both sides of the issue. (I haven't chimed in there, only because others have spoken for me.)
My position is this: aside from the legal issues, an author should do all in his/her power to support the filmmakers to make the best film version, and that includes dropping antagonistic attitudes to screenwriters, and instead being part of the team.
When comparing books to film versions, the "book was better" argument doesn't wash with me. They're two different ways for telling stories.
In this case the irate author doesn't seem to be able to accept the "kill your darlings" principle, which, I think, applies to the process of adapting, than just to rewrites.