They are rare, but the do happen. When they do they tend to be spontaneous riots and not between organized groups of fans. The famous $.10 beer night riot: https://en.wikipedia.org/wiki/Ten_Cent_Beer_Night

It might help that most beer sold in US stadiums tends to be mass produced beer with low alcohol content (Coors Light is only %4.2). Still, it's nice to have something that's not a heavy beer in the summer.