Bridge to Terebithia

So the question is: How did a book (1987) that has been used for years to help children come to terms with unexpected death suddenly become a fantasy where the children " battle the Dark Master for the salvation of the magical land they call Terabithia"?
http://movies.aol.com/movie/bridge-to-terabithia/25017/main

I had to reread this book after seeing a trailor for this version, because it seemed to bear no relationship to the book I remembered. Luckily, I have plenty of copies here in the library. I think some fantasy fans who grab it will be confused and disappointed; the book is concerned more with the running abilities and impoverished background of the main character, and with the death of his friend at the end. Hmmm.

Makes me wonder what they will do with Annette Curtis Klause's Blood and Chocolate (1997), which has been much advertised, although there was enough blood, gore, and nudity in the book to make a Hollywood film without much problem.