The issue I think centers around why the changes from the book are being made. Sure there are reasons why sections of a book could be shortened or changed to a certain extent for various reasons.
It is the arbitrary nature of the changes which are annoying. Bree was changed from a friendly welcoming town with various people for the most part living together without issue. In the movie they change Bree into a slum and the Inn into a dive. Was there any reason to do this? No. It was arbitrary. That is what is annoying. People making changes for the sake of making changes. Does it impact the general story? Probably not, but why change it? Tolkien set it a certain way, so why not respect what the man wrote and present it that way.
There are many examples of where they got so many details of the books just right in the movies, but the butchered so many others that really comes across as a lack of respect. Treebeard's character was changed significantly from the book. Why? To make the Hobbits look more important? That is not what Tolkien presented in his book. So, an arbitrary change was made which significantly changed a character in the book and ruins the character.
It is as if sections of the movie were re-written by committee, or just changed for the sake of changing something. I think that is why purists get upset. It is these changes which are made which come across as having no respect for the story the author was telling.
