I am American, so I'll make all the bazooka jokes I like thanks.
Most concert venues in America, especially those in big cities, would have a ban on hand guns anyway and it would be enforced by metal detectors and bag searches. Metal detectors and bag searches prevented the Stade de France bombers from getting inside, by the way.
It is deeply offensive to suggest that the Bataclan victims, or anyone else, could have prevented what happened to them by acting differently. But there has been a lot of offensive nonsense posted about Paris in the last ten days (and I include that crazy thread here in Preppers that tried to claim Paris descended into chaos in that - I was in the 6ieme all weekend, every shop and restaurant and bar opened as normal and far from panicking or being cowards, Parisians incredibly bravely woke up and immediately began getting on with getting back to normality).
Please don't think that every American thinks that carrying a glock in your pocket is a good idea. Many millions of Americans support much greater gun control, and there is an ongoing debate about the laws.