Asita said:
Lucas isn't lamblasted because he changes Star Wars. He's lamblasted because the changes tend to make the scenes worse.
Thread won on first post, you sir, get a muffin!
Anyway, basically what Asita said, DLC adds to a story, it doesn't generally change things, but gives us new stories and new characters. DLC in the Star Wars universe would be Lucas making a quick one hour movie that showed the Rebel Alliance leaving Yavin IV and followed their adventures as they hid from the Imperials until they landed on Hoth. Not necessary, not changing anything that's already there, simply adding a bit more to the story.
No, what Lucas does is go back into a film that is well loved and alter things that the audience loved, and thus take it away, It would be like if Valve suddenly went back and rereleased Portal but now Chell is voiced by Angelina Jolie and GlaDoS is a man. Those are the kinds of things Lucas does, and then he throws a temper tantrum when he's called out on it by his, admittedly justified, enraged fanbase.