My guess is the reason an insurance company would insist on a no guns policy is that if there is a gun accident the one they insure would probably be liable if he did not have that policy but not liable if he did. If on the other hand there is a crime committed on the property, even a mass murder, and it could be demonstrated that if customers had not been disarmed the crime would likely have been stopped (not an easy thing to establish), the property owner would still probably not be held liable for any damages for having disarmed the victims (even though he should be). This is pure assumption on my part but it does make sense to me: insurance companies are known for being coldly pragmatic in regard to their bottom line; money matters to them, lives don't.
If my assumption is correct, a state law could be written that would change the situation. For example, if a crime is committed against disarmed customers the property owner can be sued by the victims, or their survivors.
