It is entirely on topic to compare the moral responsibility a property owner has to a single employee's safety vs. the safety of everyone. You want the untrained employee's desire for his personal safety to override the owner's moral responsibility to protect everyone from negligent death.
If an employer had the ability to control the training and equipment of armed employees it would be different, but his only choice is to endorse any fool endangering the workplace with their incompetence, or allow no one.
Given only those two choices, the only moral choice for an employer is to protect the greater group of people.
One person who wants to carry is a special interest. That special interest should not trump group safety on private property unless the owner wants it to.
If this was about soldiers, cops or any other category of person who's competence was certified, it would be different. Foolish people with guns ARE dangerous.