...smart thing for a business to do is to ... hire "security guards" and have them bonded and insured, trained, and psychologically assessed.
Ok, so here's another option we both agree is smart. Let bonded, insured, trained, psychologically assessed employees carry. That would, admittedly, set the bar high, and severely limit the number of employees who could carry, but it's still not a blanket ban.
So that's a third option instead of just the two that your earlier post implied were the only choices. I imagine we could come up with more if we tried.
Second, while you are correct that a company has to be concerned about liability, and that concern is often used as an excuse to blanket ban carry, there's more to it than that.
I know this is true because the TX parking lot law gives a company civil immunity if a person who legally has a gun in their parked car in the company lot under the authority of the parking lot law causes damages. In spite of that fact, the large corporations in TX still strongly opposed the law--so strongly that they killed it the first 2 times it came up.
So liability may be a concern, but it's clearly not the only one. What's odd is that the corporations that fought and killed this law repeatedly before it was finally passed were never able to articulate why they were opposing the law past repeating the concerns about liability--even though the law came with the liability relief from the very beginning.