James Berardinelli has a good essay about the PG-13 rating, pointing out that the lives of most teenagers are R-rated, from the language they use to the experiences they have. Nevertheless, since teenagers are the main audience at the multiplex, the whole movie industry engineers its product to fit within the arbitrary PG-13 guidelines:
The insidious thing about this is that movies that should be rated R are emasculated in order to get a PG-13 rating. Nudity is obscured, sex becomes implied, and no more than one “fuck” is allowed. (How dumb is that? Say “fuck” once and it’s a PG-13; say it two or three times, and it’s an R. I know that if I’m in a PG-13 movie and someone says “fuck,” I can relax safe in the realization that I won’t be subjected to the word again until the movie is over.) People can be murdered, but their deaths can’t be bloody. It works the other way, too. In order to avoid a PG rating, some filmmakers intentionally add profanity, mild sexual content, and a little violence to attain a PG-13. Sometimes, deletions represent ornamental (as opposed to substantive) cutting. But there are times when they genuinely impact the director’s vision and/or the viewing experience, when the artificiality of how a scene is shot or edited call attention to what’s going on.
Though they follow the PG-13 rules slavishly when making a movie, and produce a crappier product as a result, movie makers skirt those rules for merchandising. For example, the producer of Avatar recently explained how his company was able to make deals with toymakers to create Avatar toys for kids under 13, including Happy Meal toys, even though kids who eat Happy Meals are way too young to see the movie.