If someone is shot on business property that disallows individual carry, then that is the business owners fault. You disarmed me leaving me helpless as a condition of doing business. It was your responsibility to keep me safe where I would normally be able to do so myself.
Fail in that duty, and I'll suit your ass into nonexistance.
There have been so many private property owners sued for setting conditions on their own property, like not allowing guns...so go ahead, sue.
Anyone can sue for any reason. Some never win though.