Actually, he's not a jackass. You just don't understand where he's coming from.
Moore actually praised the movie script as the closest he can imagine anyone getting for a film adaptation.
It's not that he's against supporting the material he has created. He believes strongly in supporting the comic book medium. Therefore, he thinks movie adaptations of comic books, especially ones that serve as a retelling more so than an original story, are no more than commercial or a dumbing down of the material. That's hard to argue against. Lastly, they are his stories as the creator, so who are we to judge his opinions about whether or not a film adaptation is appropriate. I don't think he has attacked other writers for their beliefs.
Medium is important and makes a difference in communication, in other words.
Moore earned his comic "God" status with acclaim many good comic writers haven't achieved, so give him his due and respect his individuality. It's admirable that he has unwavering principles.