Independent and foreign film companies are not part of the Big Six, which means they're completely at the mercy of a regulator owned by their competition. The Big Six dominate American film like ACME dominates the roadrunner trap and TNT markets. If you're an independent film trying to explore human sexuality as an art form (which would actually get a fairly low rating in Europe), you'll have a harder time sliding in under the dreaded NC-17 bar than an Eli Roth/Quentin Tarantino crossover film, provided the latter is produced and backed by one of the major studios that run the MPAA, of course. Hell, even Clerks got an NC-17 rating when it was first released as an independent film, but after its sequel went through a major studio, it got an R.
The Weinstein Company/MGM
Remember? The one where the dude fucks a donkey?
European rating systems typically give violent movies high ratings, but MPAA raters consider the murder of dozens of human beings via machine gun to be perfectly safe for a 13-year-old, as long as there's not much blood. But two girls making out? That shit is dangerous. Violence can be traumatizing for kids, but we'll let them watch Heath Ledger pencil a man to death because hey, it's PG-13! Meanwhile, should Heath choose to kiss a man, well obviously that shit deserves an R. But hey, the nightmares and sleep disorders that little Billy is suffering from after a summer spent bingeing on violent blockbusters surely don't hold a candle to the horrors that would await him if he saw two dudes consensually makin' out.