There's always a scenario to support or attack a particular course of action, subject only to the creativity of the person crafting the argument. One could just as easily make the case for disarming security by coming up with a reasonable sounding scenario "proving" that a security officer could shoot someone and create liability for the company and thus eliminating any armed security anywhere due to fear of corporate liability.
Having an employee go to his car and arming himself is far too easy. If he has to drive home and arm himself, that's a good thing.
You're mixing two things (liability limitation and prevention) which really confuses the issue.
Your earlier assertion was that the policies were in place to limit liability in the event that an employee breaks the law. I pointed out that liability was prevented by the TX parking lot law and yet the corporations still strongly opposed the passage of that law. Your response begins by stating how the policy actually reduces the chance of an incident (amounts to prevention) and then in turn uses that argument to support the liability limitation argument.
The problem with the idea that the policy is equivalent to prevention or even amounts to a significant deterrent is that if Don is undeterred by the "no guns in the store" policy then it makes no sense at all to pretend that he somehow WOULD be deterred by a "no guns in the parking lot" policy.
It's like saying that a person bent on murder would somehow be pacified if he couldn't get his hands on an 11 round magazine and was forced to make do with a 10 rounder, or that the OKC bomber would have been stymied by more restrictive truck rental laws. Clearly not a logically supportable argument.