I believe this derives from the many immigrant groups that have come to the US over the centuries, each with a very different cultural view of what constitutes "correct" behavior. The only way to avoid constant fighting in that situation is for most people to say, "OK, as long as you are not directly harming me, you do what you want and I will ignore it."

Europe, by contrast, is far more ethnically homogenous than the US within individual countries. Most people in most places share a very similar cultural background and very similar views of what is "correct," so the pressure to conform is greater.