Actually, F Bombs are in PG-13 movies. They're limited as to how often they can appear, but they do appear. You have to take into consider how the word is used. When does it step over the line from "mere" profanity to utter vulgarity?
Sorry. That's the best I can do by way of explanation.
You can lead a horse to water. But you can't make him get down on one knee and do an Al Jolson impression!