Contempt is too harsh a term, annoyance with parts probably comes closer.
And then, this site was launched before even the first film premiered. If you look back into old threads started at the time the films came out, you will find discussions between the film (only?) fans and the "bookies".
@Maeglin was in the thick of some of those ancient discussions, and @Squint-eyed Southerner has been digging them out like Dwarves going after mithril. That the current tendency may be more critical of the films probably has to do with the film-only crowd moving on when there weren't any more Middle-earth moving pictures forthcoming. So perhaps some time after the extended edition DVD of "Return of the King" came out in 2004, or the EE of "Battle of Five Armies" in 2015.
It's not the impossibility of translating some parts of the book to the screen, when the producers rightly assumed that a substantial part of the audience would not have read it up to that time - some decided to read it after seeing the films. It has to do with Peter Jackson turning some parts of the book on their head. Like the Ents in the film deciding against an attack on Isengard after the Entmoot, while in the book they decided for the attack.
I've posted this before. My wife and I went to the cinema to watch "Fellowship" in early January 2002. We went to see it again maximum three days later, as I had spent the entire 2¾ hours - plus an intermission - the first time screaming in my head (and occasionally grumbling audibly) "wrong, wrong, wrong, WRONG!!!" When we went the second time, I told myself "forget that it's supposed to be based on JRRT's book". Very difficult.