They do, but not that scene, and they typically stop performing the scene when the director yells cut. On this day though, the scene ended, and a dude decided to play with a gun. That's just my opinion of the event though, and I haven't read anywhere near enough about it to make a real decision. From the little I did read though, he wasn't even supposed to draw the gun in that scene, and the actual shooting took place after the scene was over.

Had he accidentally pushed over a prop house while rough housing and it crushed a person, would we be blaming the carpenter? Its definitely possible, but I think goofing around and playing with weapons should never be tolerated.