The reason they have this corporate policy is easy to explain. Could you imagine the law suite if a company employee shot a person in anger while working?
It is better to have a corporate policy that denies employees their second amendment rights than to leave it to the ambulance chasing scum bag lawyers.
Understanding the policy is different than pointing out that their decision to keep her on was more likely a effort to control bad publicity rather than any genuine concern for the employee. Having managed quick service for twenty five years I dare say I have a thorough understanding of the liabilities involved.