If what you say is true legally, then the property owners very act of allowing anyone to enter the property would subject them to liability. Like I said, without specific and actual knowledge of a particular impending criminal act, a property owner owes neither the duty nor carries the liability to protect the public from the unexpected criminal act of a third person.
Here's my idea: Businesses will no longer be allowed to forbid the possession of firearms by the general public. BUT, we will impose an absolute duty on anyone who CCW's or possesses a firearm. They must use thier firearm if they become aware of a criminal nutball who is shooting other people. That means that you can't leave, wait for the police before acting, freeze in fear, or hide. You MUST confront the nutball and engage him or her in a gunfight. You absolutely must do this, and you now have a legal duty owed to the public to do this. If you fail to engage the nutball in a gunfight, then the victims (or their families) who are shot by the nutball can sue the CCW-er for failing to protect the injured or killed.
I don't seriously endorse this idea, but I'm trying to illustrate a point. Imposing a duty and liability on others to protect unrelated members of the public against the unexpected criminal acts of a third party is a bad idea.