The problem is that it is not fair both ways. The corporation faces virtually no liability for an employee killed during a crime so it isn't balanced.
True, but "fair" and "balanced" don't really carry much weight when the matter gets heard by the courts.
In the 1990's, the gun control lobby got to corporations, especially the human resources culture, in a big way. An armed employee in the workplace was a ticking time bomb, just waiting to go off on a rampage. Allowing employees to have guns in the workplace was going to result in casualties in the thousands. Watch Bob in accounting to see if he discusses guns; that could be a warning signal. Report Dan if you catch him reading anarchist literature like American Rifleman
In the end, it comes down to the concept of liability. Corporations are convinced that an employee carrying a gun is likely to have a negligent discharge or shoot the wrong person. That sort of stuff keeps CEO's up at night and lines the pockets of the lawyers.
On the other hand, if an employee gets killed by an armed robber, the corporation just loses some property. The all-powerful concept of liability is avoided, however. Is it right? No. Is it fair? No.
But that's where we are. And they have better lawyers.