Even the MPAA disagrees with you. While movie ratings have become more lax, each of Abrams movies is a PG-13 rating. With the exception of Nemesis, no movie was over a PG rating.
(1) MPAA ratings from different decades aren't comparable.
(2) MPAA ratings from the same era are, often, not comparable.
(3) Star Trek: First Contact
was also rated PG-13.
(4) The PG-13 rating didn't exist until July of 1984, so the first three TOS films couldn't have been given it (and obviously weren't going to be rated R).