I think "no shirt, no shoes, no service" is more like asking customers to not come in with an AR slung over their shoulder, but to instead carry concealed. I have no issue with a business establishing a standard for something visible which impacts the business environment.

Also, wearing a shirt isn't likely to cause the customer harm in the event of something unexpected happening-- and if somehow it were to, I doubt any business owner would have a fit if someone took their shirt off if it caught fire. You don't have that kind of safety option when the sign is "no guns" and you suddenly need one that you can't conjure up out of thin air.