It should be. At least partially liable for damages.
Or would you take the position that if a person is going to break the law with a gun, there's very little you or a security guard can do to stop him.
Not if you are the first victim, and have no warning that something is up. Otherwise, you could shoot the SOB and save yourself and any other would be victims.
I don't believe you can hold a business liable for someone else's actions.
You don't, you hold them liable for prohibiting you the means to protect yourself, if they are not willing to do so.
Btw, I don't have a conceal permit but I am always hoping someone around me does when I'm out and about. So here's my "thank you" to those of you who are legally packing!