You guys have to understand that ever since the initial riots a few years ago in East Lansing, it has become sort of a "tradition" with MSU students in one section of town. It's like the people always talk about how crazy the initial riots were, and nobody experienced it, so after the games people kinda horde in this one section hoping to see something. Add in plenty of alcohol, and plenty of police on hand (with riot gear ready) and there is inevitably going to be some incidents that occur.
I was there 2 years ago for a mini version of the annual riots. My buddy lives in an apartment right at the site of where it all goes down, so we basically got everyone we knew there and had a keg and whatnot. There's nothing quite like drinking a nice cold beer and sitting on the back porch while watching 8-10 cops in full riot gear start trying to settle down the crowd. Fortunately they didn't gas everybody that time, but according to my friend his apartment still faintly reeks of tear gas from the last game.
Basically, people just do it because they want to be a part of the hoopla. You have to remember that MSU is a pretty big party school, and the average student that goes there would be thoroughly entertained by watching their fellow students get tear gassed. I'm assuming 99% of you guys would be watching/rubbernecking in awe seeing 10 cops in full riot gear parading down your street.