Lucas has admitted that he doesn't like directing that much. The only sweeping reason that I can see that he should be considered an auteur is that his movies (as a younger filmmaker) deal with issues of the innocent individual versus the dominating society. THX1138, American Grafitti, and Star Wars all have this theme to some extent.
Not sure if this makes ESB and ROTJ the works of an auteur filmmaker, though, since they were not Lucas' films as far as directing goes.
Also, it seems that his "auteur-ship" comes more from his story ideas than his directing, which seems pretty straight-forward and un-stylized to me.