You misunderstand. In order for liability to be established, you would need to prove that whatever injury DEFINITELY WOULD NOT HAVE HAPPENED if the victim had been allowed to carry his gun.
The plaintiff has to prove no such thing.
While we can't really go through the issues without the facts, the plaintiff will only have to do a prima facie case before shifting the burden to the defendant to show he fulfilled his duty. (If there was one. In our current scenario, I assume invitee.) The duty is to protect the plaintiff from known harms. If the place is getting knocked over repeatedly and the owner does not allow customers to carry weapons, if one were hurt from this known harm the property owner would be liable if they didn't satisfy its duty of reasonable care.
That gets to be a fact balancing issue. If the business used buzzers to allow people access to the business, had armed guards, video surveillance and the like, the fact he did not allow concealed carry might not be a big deal. On the other hand, if the owner did nothing but prohibit concealed carry even though he knew another robbery was reasonably foreseeable (due to the other ones) he could be liable.
As to the reverse, where we make the argument that a bunch of armed people running around a business might increase the chance of an innocent getting shot, it is irrelevant. The legislature gave those who allowed CC immunity.
