i am in no way defending this movie or any other... but it's a LITTLE more complicated than simply choosing to stick to the subject material. most successful movies based off books are based off of short stories because it's much easier to ADD to the story in a meaningful way than to trim. and unless you trim a book, even a 200 page book is going to end up 6 hours long.
so the problem then is that when you trim down a book to fit in 2 hours, you have to cut out a bunch of stuff that the original author thought was important enough to write about. you take that stuff out, because you have to, but lose a lot of the soul of what made the book special. then there might be aspects of the main plot that don't make sense anymore, so you have to re-write it in a way that explains why these characters would behave differently without having had the 40 page sidequest where they learned the true meaning of friendship or whatever nonsense the book is about.
christopher nolan makes a lot of his movies based off of short stories, so when he adds something, he gets to add his personal flair to it, instead of ripping huge chunks out of someone else's soul
