The MPAA is a running joke. Go watch
This Film is Not Yet Rated.
There are also some great articles about the farce of negotiating cuts with the MPAA to get ratings. I remember Trey Parker and Matt Stone had some hilarious stories about what they cut to get the South Park Movie down to "R" but left in
or even added that was much worse than what was cut.
With respect to these PG-13 movies, I imagine they are actually much tamer than the trailers let on. I also find this with comedies. If a comedy tries to play off that it is of the Apatow/Todd Phillips genre and gets a PG-13 rating, it is usually is pretty lame. But, for instance, some of the stuff that stupid Transformers 2 movie got away with would never have flown without an "R" rating if it wasn't MICHAEL BAY!!CGI!!MEGABUCKS!!!