I dunno, but I have season tickets for my local MLS team here, and there is zero violence. Just 20,000 people singing for 90 minutes and having a good time. The away fans say it's the best atmosphere in the league.
In terms of Spain, I also feel it's a Spanish culture, too. Portuguese are the same way. Going to the games are like going to the theatre. People dress nicely, have their programs and handkerchiefs with them, and applaud goals and some good plays. That's generally it. For instance, look at what time most EPL and Serie A games are played compared to when Real Madrid and Barcelona play. The latter two play sometimes at 9 or 10 PM! People dress up, go out for a nice dinner, then attend the football match for the entertainment in their evening. It's just a different culture.
Unfortunately, like JA said, violence is equated with the singing and more lively fans in Spain. The Basques are the worst. Osasuna at Reyno de Navarra, or Athletic at San Mames, have violent and disgusting fans. I am saying right now it's NOT the majority(not over 50%), but the sad truth is that many act like absolute animals when Real Madrid play there. The objects thrown at the players are signs of complete filth.