It wasn't a riot. Just tons of debris for 45 minutes from thousands of fans. Nobody got violent as far as I know. MLB denied a protest and called it a judgment call. What a load of crap.
Well, it has to be a judgment call. It's not the umps doing something clearly unauthorized (which is the only time a protest succeeds), in this case, it's a misinterpretation of the effect of the rule. If a strike is low, you can't say "how can you call that a strike, it wasn't above the knees. The umpires don't have the authority to call something a strike that is below the knees." It may be the wrong call, but it's still a judgment call.
Anyway, I think the Braves fans were justified in being upset. However, you know that there would be universal criticism if these were Philly fans.