George Lucas does have and always has had creative control of TESB and ROTJ. Not only has he had legal ownership of TESB and ROTJ from the beginning (and he gained ownership of ANH from Fox in the deal to distribute the SEs), his role as Executive Producer gave him final say on any aspect of the films. As Lucas explains in the ROTJ DVD commentary, the Star Wars sequels were managed more like a television series in which the Executive Producer controls the content and the director is merely hired to do the work necessary to get it on film, whereas normally in film the director is the main creative force. Because Lucas owned the films and paid for their production with his own money, he could assign and define jobs as he saw fit.
Furthermore, Lucas resigned from the Director's Guild of America and the Writer's Guild of America after they fined him for the lack of opening credits in TESB (after the unions had given him a waiver for the exact same "violation" in ANH), so, Hollywood union rules cannot be enforced against Lucas either, since he is not a member.
The bottom line is that Lucas is 100% within his rights to do anything he wants with these films. I don't want to be called a Lucas apologist for pointing out an objective fact.