from what i remember, cops are taught something similar, if not the same rule. It's why police is extremely leery of letting someone with a melee weapon or knife get anywhere near them, and if people dont follow instructions to stop, the cops will shoot, despite public outcry about how far away he was, or he "only" had a knife, etc. The closer the distance, the less advantage the firearm has over other types of weapons and even someone unarmed. And once the distance is closed completely, I believe a knife weillder holds the statistical advantage of winning the fight.