On the one hand, they do belong to Lucas and are his to change. That can't be disputed -- it's a business fact.
Where I disagree with other fans is on the quality and importance of George's "vision". Quite simply, I believe that a talented 32-year-old maverick independent filmmaker called Geoge Lucas wrote and directed Star Wars, and his vision was great. But now a middle-aged, out of touch, eccentric billionaire businessman called George Lucas is using his vision to change the films, and to make a prequel trilogy.
The flaws inherent in the Special Editions and the Prequel Trilogy prove that this man's "vision" isn't a vision I respect very much any more. It isn't a trust-worthy guiding vision. It's a pretty lame vision, and not one I appreciate being used to modify films from a different era.
It frustrates me to see Star Wars fans revering him as the God of all things. It frustrates me that they convince themselves to love very flawed material just because it's what George made. The absolute silliest example is: "...But The Phantom Menace is supposed to be boring! That was George's vision! Therefore it's great that it's boring." Oh how I laugh. No film is ever designed to be boring... except some experimental films by Warhol.
Currently, when a Star Wars fan dares to say they dislike a certain scene, they are slapped down by everyone else on the message board and told that they are to blame for not accepting George's vision, and they'd love the scene in question if they respected George's vision like they're supposed to. But how much flawed crap will George put on the screen before his worshippers finally say, "Actually, you know what? I've just realised that if I don't think it's very good, maybe it's George's fault rather than mine..."
It really is like a religion. Seriously. A strict George worshipper arguing with someone who doesn't respect George's vision is just like a strict religious person arguing with an atheist. The religious person has built all kind of mental defenses to prevent them from contemplating the idea that God doesn't exist, and can't imagine living in a world without God. A George worshipper has built similar defenses to consider George's vision all-important, and can't imagine being a Star Wars fan if you don't agree with the vision.