You bring up a valid point
The key to this is that as you said the guy does not see you behind him as a threat. We can debate as to whether or not to draw and fire, but it does provide a good reason why not only a pistol should be carried but also a good tactical folding knife.
In this situation I would have to say that a nice little knife clipped to your pocket could be as effective if not more effective then a pistol on your hip. You have a chance to surprise him and you are close enough to deploy the knife before he could swing around and use his weapon of choice, presumably for arguements sake, a gun. This also prevents a miss or over penatrating round from potentially hitting the clerk(s) and can do catastrophic damage to the BG in this situation.
Just food for thought.