Maybe low scores and no on-field violence are the cause of your trouble, maybe the fans feel the need to make up for there being no action during the game.(sorry couldn't help myself)

Seriously though, I don't know if it's the nature of the sports we predominately play (cricket, NRL, AFL) or if we are just more relaxed here, but the kind of game related violence you guys are describing just doesn't really happen here. Like I mentioned earlier, 2 people following opposing teams can sit down together and have a few drinks while whatching their teams thrash each other, and if your team so happens to lose, it is a given that the other bloke has bragging rights and will give you plenty of about it, but you take it on the chin since you get to do the same next time his team loses. Hitting someone because your team lost just isn't cricket.