Being that their business is on federal land, the law of that land applies in my opinion. Once the shop owner dies and there is nobody left to leave the business to, where does the shop and its land go to? The government. They are only renting. Even if they buy.
So does that apply to your home as well? Anyone who wishes can just make up their own rules? Sorry, still disagree on making your own rules on others property, even if they are only leasing/renting it. If you really think that applies, have you tried to OC at say a court house or a federal building where they state no weapons allowed?