The other issue is the opportunity cost of ALL of the victims and potential victims being disarmed by the policy.

We have no way of knowing how many of the victims were gun owners that would have been armed had it not been against policy. As such maybe this woman being armed wouldn't have mattered at all, but if one of the first victims would have been armed and engaged the shooter at the beginning of his spree, how many of the 12 dead may have survived?