So I can only assume they're willing to explicitly state that they'll accept financial liability in cases where there's a shooting and someone wants to sue them and argue that their policy aggravated the loss? I mean, they estimate the risk is less this way, and repudiating BOTH risks would hardly be cricket, right?
The way I understand this, the insurers do a cold, hard calculation. Besically, in many states, if you have a sign that “weapons are not permitted”, you are implicitly promising protection to unarmed people inside. If you are unarmed, and get shot in the building, you can sue the lessor/owner of the building for “wrongful death” or injury and win. Insurance companies know what states this works in, how much a life or injury is worth there, and adjust their rates accordingly. Mass shootings are expensive in these cases, but that is what reinsurance is for.