So there's a correct basic concept in this, in that you as Joe Citizen have no duty and possibly questionable authority/justification/right to draw your weapon in a situation where your life is not being threatened, and there's some misapplied explanation, in that store policy has nothing to do with your lawful justification for doing so.
I think we can consider as agreed that in no circumstances are armed guards/persons justified in using lethal force to protect the money (goods, properties, etc.). Only taking of life itself, and felonious assaults that threaten various grievous bodily harms, arson, and kidnapping are considered lawful justifications for the employment of deadly force. And that can be such assaults on you, yourself, or on another ... sometimes.
But when using as lawful justification for use of lethal force the threat to someone else's life, there are caveats in the law. They usually say something along the lines of that you can only act so in situations where this threatened third party would be within their own rights to use exactly the same force to defend themselves.
But what about their own decision to do so? We make an assumption that every person who's ever stood in front of a weapon wielded by a bad guy and handed over their watch and wallet (or the register till) was praying to their deity for US to step into the scene and save them.
It appalls and outrages us to ever hear that someone doesn't want guns drawn or blood to be shed when some injustice is perpetrated, like an armed robbery. And yet, statistics indicate that the huge majority of armed robberies end with no injuries, so any call to conservative hesitance to introduce a second weapon and possibly trigger (lol) bloodshed actually makes quite a bit of sense. Any quicki-mart clerk who's been robbed five times before knows s/he's probably going to hand over the drawer, talk to the cops, and then and go home safe tonight. Does he or she want to roll the dice and see what happens next if you draw your gun? If you yell commands at the bad guy? If you shoot? Maybe. Maybe not.
If you step into the scene and draw your weapon, you're making the choice for him/her. You may be right. You may be wrong. It isn't store policy that should concern any of us, but whether we'd want someone ELSE to make a choice FOR US to mess with the probabilities with lethal results.
If someone's GOING to die at the hands of a violent criminal, the decision may be simple. But that's far from a foregone conclusion and one we'd be a fool to jump to without overwhelming evidence -- evidence beyond the presence of a criminal's weapon.