They usually cite insurance costs as the reason for denying their folks the chance to defend themselves - they value a few thousand a year more than they value their employees' lives. On the flip side, I can picture insurance companies "fixing" this by raising the rates for any establishment that hires workers to "cover all eventualities". Lawyers and insurance companies are extensions of the anti-freedom Left.
The state governments could fix this by changing the laws to disallow lawsuits against people who are defending themselves.
As for the clerk that drops their gun and kills a customer, there is liability that cannot and should not be excused, if the employer mandates that employees carry. That said, the state governments could again pass legislation, which limits liability to the individual, so that the employer cannot be sued.