The banks' insurance carriers figure they don't have to pay up when someone's shot during a robbery. They do have to pay up if a bank employee accidentally shoots a customer.
The way to deal with this idiocy is to sue the bank for "negligent failure to protect" (my phrase, I made it up) the next time someone gets injured in a robbery attempt. Here's the logic: an action for negligence requires four elements: duty, negligent breach of duty, proximate cause, and actual damages. Obviously, someone who gets injured has actual damages, so here's the rest of it:
1) When the bank enacted a policy effectively prohibiting the exercise of the right of self defense and defense of others, it undertook a duty with respect to its business invitees (a code phrase that means highest possible duty to begin with) to keep them safe from harm. My theory is that if there was one person there who could have been lawfully in possession of a firearm and who would have been, except for the no-guns rule, then lives would have been saved and injuries averted. But by enacting such a policy, the bank effectively took control over the defense of people in the bang, and undertook a duty to take steps to see that they be kept safe, whether by posting armed guards, arming the employees, or otherwise.
2) When the badguys show up and cause a violent altercation, the bank negligently breaches its duty to protect its business invitees by its failure to take active steps to see that they be kept safe from harm.
3) The failure to protect the customers when a violent confrontation breaks out will have been the proximate and efficient cause of the customers' injuries. The bank robbers' actions are neither an intervening nor a supervening cause, because it is the harm which is reasonably foreseeable when one goes into a bank. People have been robbing banks for as long as there have been banks, so no one can claim they didn't expect it to happen.
See how the insurance co's. like that one.