The MPAA is just doing what is required by American law to protect its registered trademark. How difficult would it be to put Fic- in front of each rating (Fic-G, Fic-PG, Fic-R, Fic-NC17, etc.)? The networks do that with the MPAA ratings (TV-G, TV-PG14, etc.), and it doesn't violate the trademark.
Of course, the others are funnier.
