I guess I won't move to the USA anytime soon. If "some stupid people start killing customers" is something that a store expects to happen any day and that they have to be prepared for. However, in this case, had the robbers arrived a bit later when the shop was open and customers had actually been present, the forty shots fired could have easily killed a customer or two or some employees, while an unarmed guard would have let the robbers escape with some hardware and without a shot fired.
Of course, with America being the world's freest country, and free speech and all, you wouldn't want anyone there actually taking advantage of their right of free speech.