Erikstormtrooper said:
I think LFL's issue is that Prowse writes "David Prowse is Darth Vader", not just "David Prowse".
In that case, I understand Lucasfilm's objection, although I do not agree that Lucasfilm should be empowered to ban him from writing the slogan. "David Prowse is Darth Vader" sounds awfully like he is minimising the contribution of other artists to the character. Why not just sign it "David Prowse"? No one could justifiably object to that; it would identify him with the character without sounding as if others were being sidelined, and everybody would be happy.