My theory on why Alan Moore does this:
1. He feels he has a reputation to uphold as a 'curmudgeon'
2. He wants to publicly dismiss the movie version of any project he's in because its a win-win situation. If the movie sucks, people will say 'Moore was right'. If the movie is awesome, people will say, 'As good as it was, it STILL wasn't what Alan Moore thought it should be...he MUST be a genius!'
My other theory on Alan Moore
He's a very good writer - he's got an extremely clear creative vision, and that's rare in any medium. He's possessive of his products, and who wouldn't be if they produced what he has?
I think these two theories interact with each other to produce interviews like this one. I have tremendous respect for him, but at the same time wish he'd go on record explaining things that he likes, and why he likes them... not just why everything is subpar... Its happened, but so rarely.