There's a prudish streak in the US that I really believe comes from the fact that the country was originally settled by people too religious and too concerned with overtly moral social interaction to be tolerated in 17th century England.

That may seem like too easy an explanation but I really do think there is some truth to it. Despite the excesses in American culture, that sort of casual swearing still gets backlash when it goes beyond people who are familiar with each other.