Characters from Books to Movies

Ok so we saw Eragon this weekend. As I said before it overall was an ok movie. I have a problem with how the lead female character was portrayed. Now I am no feminist here by any means but is there something in Hollywood which states that any strong female character from a good book must be turned into a toady and lackey for the lead male character once the story hits the big screen?
In the book Eragon Arya (an Elfish princess) is strong willed of few words spoken and very distant when dealing with normal men. She is a fierce warrior and powerful magician. In the movie she is pushed into the role of fawning all over Eragon and following him around like she has a crush on him. It is completely opposite in the book, there she rebuffs Eragon romantically and would never follow a regular man and grovel like she did in the movie. This leads me to another character in a recent movie, this one a blockbuster, who was treated in the same fashion.
The DaVince Code, based on the novel by Dan Brown, did the same thing with the lead female. Sophie Neveu a noted cryptologist and independent as can be in the book is resigned to whining to Tom Hanks and again becoming the toady and tag a long to Robert Langdon. In the book she almost uses the character Robert Langdon to find the murderer of her Grand Father. In the book it seems like he has to drag her along to find out the secret of the Holy Grail.
Why does Hollywood have to change the actual characters and who they are for the movies. Other examples of this also come from the DaVince Code. First of all the Bishop Aringarosa did not know that the albino monk Silas was murdering people on the Teacher's behalf, in the movie he actually endorses this (no wonder the church was so upset about the movie). Then the Detective Fache figured out what was going on and was trying to save Sophie Neveu and Robert Langdon but in the movie he seemed to be hunting them to try and save Opus Dei.
I mean come on Hollywood there is a reason a book becomes a best seller and that is because the storyline is well written and the characters and their roles become vital to the outcome of the story. So stop changing who the characters are, I understand you may have to tweak the storyline to fit it into a reasonable amount of time but that does not mean you have to destroy the relationship between the book and the movie.