IF they prevent employees from protecting themselves then haven't they failed to take a reasonable action to protect their employees?
It depends. What is reasonable to expect? Would a jury of 12 find that need for an armed response was a reasonable expectation during the course of an average shift? If you were a night clerk at a 7-11, maybe so. If you were a night watchman, maybe so. Bank employee, diamond courier, etc.
OTOH, the employer may be found to have endangered his employees or customers by allowing firearms. If you were a private grade school instructor, they may decide that the employer was negligent in allowing firearms where kids might have access, if one of the little darlings got into your glovebox and shot himself with your glock.