Bottom line, the deli is private property. The deli owner makes the rules. Even an alleged conservative should respect property rights and either mask up or go somewhere else. It's not the deli owner making the rules. It's the state making the rules based upon dubious public health policy. If you're looking for the property rights violation in all this, it's not the customer who who refuses to comply, but the government that demands the compliance. So just mask up "conservative", after all, it's been ordered by your governor outside of the legislative process, all to protect you from yourself.
https://www.mass.gov/news/wear-a-mask-in-public
The deli is still private property regardless of who's making the rules. As for the rule of law, as much as we may not like it, at the state level these sorts of (perhaps stupid) regulations in the name of public health are enacted all the time. I share with you skepticism as to the validity of such regulations, but it remains that the state and in some cases local municipalities have that power. Finally, we deal all the time with stupid laws, i.e., prohibitions on carrying firearms, but in general we don't pack heat illegally.
The massholes lecturing the deli owner, to be sure, are misplacing the direction of their vective. But the sanctity of private property should still be respected...you don't like masks, go to NH.