The stories were written by Lucas. The characters were created by Lucas. Everything was done with Lucas's approval. If you want to begin to say he merely 'owns' ESB and ROTJ, then you really have no compassion for the artist. These were George's films. When he wasn't physically able to actually shoot them, he chose the people he thought would best tell his story the way he wanted it. They did so. I don't think Kirschner for one minute believes ESB is his film, his vision. I don't think Kasden for one minute thinks that this was his story. They did the jobs, technically.
They were paid to do their jobs, like a cameraman or a grip is. Do you think the costume designer should have a say in way George wants his films to look? It's basically they same thing.