Greg Cox wrote:
STAR WARS and CLOSE ENCOUNTERS had somewhat reversed that trend by demonstrating the box office potential of more family-friendly sf.
Lucas was worried ANH getting a G rating would make the movie look uncool to teens, so he put in the burned bodies of Luke's uncle and aunt to get a PG rating.
I don't have the book in front of me, but I believe J.W. Rinzler's The Making of Star Wars
says the rating was the studio's decision (Fox). It was them, not Lucas, that ensured the PG-rating from the MPAA. Anyone who has the book care to check?