A pizza delivery man for Pizza Hut recently used his own gun to defend himself against a robbery by shooting the robber. He was not charged by the police, who confirmed that he was defending himself. He was later fired by Pizza Hut for carrying a gun against company policy. The first reaction is to see Pizza Hut as heartless and stupid.
The pizza companies are merely doing what the society wants, as expressed by the liability law. If they allow drivers to carry guns, then that is a corporate policy of allowance. They would be supporting the practice. Then, if a driver caused an injury with a gun, there would be a big lawsuit against Pizza Hut, which they would lose, if they couldn't prove that the third party had threatened the driver (and maybe even if they could prove it).
On the other side, some people think that Pizza Hut should pay damages if a robber injures one of their drivers, because Pizza Hut should protect their employees. Never mind that Pizza Hut has no power to issue guns to its employees, and it isn't clear what protection could be offered otherwise.
I think that Pizza Hut had to fire the employee, or some later lawsuit would claim that they condone their drivers carrying guns, even though they have a policy against it.
We have a society where companies are held liable for all bad outcomes, regardless of the good sense behind a policy. It seems that the best a pizza company can do is to hope that their drivers carry guns, hope that they don't find out, and to fire any driver who uses a gun even in self defense.