Blowing yourself up is still something that you watch more than participate in. Sure, if it was real, there would be collateral damage, but within the context of a metal show, it's just spectacle.
Someone "pretending" to spray you with an automatic weapon doesn't give you the choice to just watch. You react, and I would think that the reactions were not universally positive. It seems like a completely stupid thing to do in the name of entertainment. If that's not what he was going for, and it was another example of Waters making a point, or whatever, then this just confirms to me what an asshole he is.