I haven't seen the movie yet. Here is what IMDB website had about the rating:
Rated PG for language, some suggestive content and momentary teen smoking.
http://www.imdb.com/title/tt0427327/
There is a parent's guide if you skim down.
I think every parent has the right to limit what their child watches. Some are more sensitive and bothered by different things. What bothers me, might not bother you... etc...
I read a Christian website review about ALvin and the Chipmunks. It made a big thing about the chica bow wow scene. Of course, this is the only line in the whole movie my 3 year old girl cares to repeat and even refers to the movie as the chica bow wow movie... I laugh and go on, but it might bother someone else.
I would just explain to my children how they will be faced with "my friend's parents let them do it" stuff until they are 18. You understand their frustrations but you have to do what feels right for your family. My oldest is 13 and I hear it all the time and have for several years now. I hate that we all don't have the same standards but that is what makes the world go round. Most likely, the friends' parents didn't realize those scenes were in there and just hoped it was over their child's heads too.
Good luck!