The American rating system has no legal binding. The reason it is so well-enforced is due to major theater chain's agreements with the MPAA. They agree to card for R rated movies, and not to show unrated movies, which in turn causes all major studios to get their movies rated, which costs money, and creates the business. As the MPAA is a private business, any legal enforcement of the ratings would be severely breaking anti-trust laws, I believe. This is why independent movie theaters, and many video rental places do not card at all, because they have no obligation to.