It is because of Amerca's obsession with sex and any "taboo" subject matter.  We should all face the facts that part of America loves sex and part of America loathes anything sexual.  The part of the population which thinks any interest in sex, other then for reproduction, is morally perverse.  They are motivated by puritanical religious mores and push the government to limit the public's exposure to anything erotic.  They try to control sexual acts, speech and thoughts of society claiming they are "protecting" American values.  Which begs the question...when did enjoying sex become such a "bad" thing? (which I will address later).  But when prohibitions are put on anything it piques interest in that subject.  The first thing recorded after the movie camera was invented was, you guessed it, people having sex.  There were the notorius "French Postcards" during the Victorian Era.  And when we try to limit erotica, as is happening for the last 20 years since the Meese Commission Report, the interest in erotica blooms.  People then want to see this "taboo" subject matter for themselves.  In Europe the attitudes are much different about sex.  In many european countries prostitution is legal, and there is much less "moral outrage" over pornography and eroticism.

American students are taught about the Puritans in Colonial America.  They read stories about "Scarlet Letters" and the Salem Witch Trials.  The Age of Enlightenment dawned and Puritanism fell out of favor.  Even though Puritanism was discredited before the American Revolution it still is part of the national subconsciousness.  There are some in America today who romanticize the era when alleged witches were burned at the stake and adulterers were publically disgraced, humiliated and jailed.  As a society we have never fully given up on the ideals of the Puritans.  And anybody who "enjoys sex too much" still must wear the Scarlet Letter. 

Reminds me of a comment Chuck Palahniuk (or somebody else associated with the movie) made about the outrage over Fight Club. He contrasted the uproar over the violence in Fight Club, even though only one person dies in the whole movie, with the free pass that the latest Bond movie, or something similar, got, even though that movie had an incredibly high (though bloodless) body count. His point was something to the effect that the movies (and society) have no problem glorifying violence, but when portrayed in a realistic and horrific manner, people start protesting. Can't pop the nice little bubble of naivete people want to live in.

and allow you up to two uses of "fuck" as long as the word appears in a non-sexual context. What kind of logic there is to that, I really couldn't say.

Matt, I couldn't say either.  No kind of logic.  It was a perfectly good four letter word for a sex act which has been degraded by overuse in a non-sexual context.  I have to admit that I use it myself, and mostly in a non-sexual context, so I can't very well be the language police here.  And three uses of the word in a movie would give it an "R" rating. This must drive the writers nuts.   Thanks for the inside scoop.   

The MPAA is bullcrap.  An R-rated movie is for adults, kids 17 and under can't even get in without an adult, they should stop trying limit how much gore people can see or how many f-bombs adults choose to hear. I work in the movie biz here in Hollywood, I'm 27, I remember getting into Friday the 13th movies when I was a kid - with my parent's permission. Didn't faze me at all, and in fact was a ton of fun. Rememer the Nazi's face melting in "Raider's of the Lost Ark"? You don't see that kind of gore in mainstream movies anymore.
It's like the scare over marijuana, just a bunch of bogus moralizing.  If you don't want to see an R-rated movie, or you don't want your kids seeing them, then don't go. But don't make me, as an adult, have to see toned-down, non-violent, weak-kneed action movies.  Hell, "Total Recall" would probably get an NC-17 rating if it was released now.
R used to be for adults. Now it is for adults who can't handle real life.

I think being a parent has a big impact on how most people view the rating system.  Before I had kids, the rating system seemed ridiculous to me.  Now that I have kids, I'm thankful there is a system in place that gives me some clue as to the content of a movie.  My kids are still young.  I want to know if there is going to be cartoon violence or realistic violence in a movie.  Some people might feel that kids need to be 'disturbed' by violence.  But I'd rather know that if I go to a movie with a G-rating, it's unlikely that my kids are going to have nightmares for the next week.  I appreciate the filter when it comes to profanity and sex, as well.  Now that I have kids, I get the difference between PG and PG-13.

Question:  Just how 'disturbed' do you want your children from watching a movie?  Night terror scared?  Just wetting the bed scared?  Scarred for life scared?  Help a brother out here.

I remember reading that David Fincher had to recut the scene where Ed Norton beats Jared Leto to a bloody pulp because the MPAA thought it was too gory for an R rating.  The recut scene showed less of the actual fight and more of the onlookers' horrified reactions.  Interestingly, Fincher thought the new cut was more difficult to watch and more disturbing but the MPAA was happy because it showed less blood.

I wish that after seeing a war movie the thought of going to war would be huanting enough to at least demand a second look at bogus reasoning.  It is not an exageration to say that Hollywood violence is largely responsible for American eagerness to kill.  What could have been an effective tool in relaying the actual horrors of violence has instead been squandered by producers who know Vin Diesel couldn't sell a single ticket unless more than a handful of people are shown being killed.