Well, the instance of the F-Word has also been an MPAA thing for quite some time. Take the great comedy movie of "Plains, Trains and Automobiles" a good, funny, movie starring Steve Martin and John Candy. It's rated R. Not because it was trying to be a twenty-first century raunchy sex comedy in the 1980s but because of one scene in the movie where Martin gives an F-Bomb laden rant at a rental-car counter.
PG-13 movies are allowed one use of the F-Word and even then if it's used in a non-sexual context.
These little "rules" are certainly odd ones but it's what it is because parents don't want their 13-year-old kids to go see a movie and have their ears melt from hearing the F-word twice.
TV has a similar oddity when comes to censorship, see an infamous bit with George Carlin talking about the problems of saying "piss" on broadcast TV. Like movies and the F-word you CAN say "piss" on TV. But you can't say it in the context of the bodily function/fluid.