Nothing new about robbing someone who is openly carrying a gun. This sort of thing was a fairly regular occurance when I was an armed guard on a Brinks Armored Car in NYC. Especially when our arms were full of bags of money or opening a door. Happened a lot to the Loomis and Wells Fargo guys too.
Fact is the presence of our guns had almost zero deterrent affect on someone willing to take the risk, or who thought they knew the routine and where to spring the ambush. If you were alert, prepared and lucky, you survived it. If not, the other guards on the other trucks tried to learn from your mistake(s).