Why aren't American sports fans more violent...?

This is most likely associated with the general trend. Here in the United Kingdom people become accustomed to associating football with violence from the early visits to matches, so it is considered as a good excuse. Obviously in the United States sports fans are more used to visiting such events for the purpose of watching the game itself. This is evident by the fact that Rugby matches here do not attract violence, despite the fact that are often held in the same towns.

I don't believe that we drink less at sports games. I believe people enjoy the games more here and don't see a reason to be violent. It is not like getting violent is going to help your team out any. I don't understand why a true fan of the game would want to ruin it by being violent.

They "luck" the spirit of fanatism. It is simple. They can be fanatics about the music and stars but for sports... they seem to find it boring.

Fear of the bigger nut. A lot of people are worried that the other person has less to lose in getting into a fight, and is too scared to provoke one.