the fact that no one is saying he can't edit his movies,
We both know that that is not true at all.
I've never met anybody who cares about the changes he has made or even future changes he might make. Doesn't mean everybody likes the changes, but if he'd release the original along with his new version, everybody would be happy.
I think this belief stems from the ambiguity of the word "should". I certainly say "he shouldn't make changes to his movies" all the time, but I really mean "It's not advisable that he make changes to his movies (because he always makes them worse)". But certainly some could misinterpret such a statement to mean "It it morally wrong for him to make changes to his movies" This is compounded by the fact that people (including me) definitely feel it's morally wrong for him to suppress the original films. A "should" in one statement right next to a moral issue in the next sentence is pretty much asking for a misunderstanding (and that's if you even bother to separate the two issues into two distinct statements, which most people don't).
So I can see how Alexrd (and perhaps Lucas himself) thinks people are saying he can't edit his movies. It's a sad fact that language is ambiguous, and that leads to misinterpretation. They are still wrong, but I can understand how they got there.