I think you have hit on why we disagree. I believe that it is the customer's duty to provide security and not the business. Even if the business disallows guns, the customer's duty to protect himself requires that he either ignore the prohibition or cease doing business where he cannot protect himself.
I don't see the moral or ethical mechanism that transfers the duty from the customer to the vendor.
Neither did McDonalds. And because of that - they lost a $27,000,000.00 lawsuit.