According to Neil Gaiman, the news piece about Alan Moore being unhappy about Constantine movie decisions is not entirely true. Yes, Alan's removing his name from the film, but it's because of some frankly rather insulting legal issues involving the LOEG movie - I think he doesn't want to risk being put in that kind of situation again.
This explanation makes more sense to me than the previous one. From what I've read, he really isn't bothered by changes in adaptations of his films at all. Look at the massive changes made in From Hell and League - why would changes in Constantine cause such a reaction?