By black letter law it is no different than refusing service to a wheelchair bound patron. I guess they should stay home too. And I’m sure there are resources to investigate federal crimes.The only reason for a person not to have a mask aside from being careless is to have some sort of medical condition and that person
most likely should ask for help and stay at home in the middle of an epidemic. With the epidemic all sort of posibilites are there that were not there before in terms of getting the foods or items someone needs.
The State will have a heck of a time investigating every possible complaint of all businesses demanding a mask in their facilities and then associate that with some sort of discrimination when we are in the middle of an epidemic.
We are not talking about not allowing people wearing pink tutus but everyone trying to follow the very same guidelines people are following all over the planet to deal with the situation.
In some countries all the goverment has to do is to ask (like japan, Norway, Taiwan, etc...) in some others they have to enforce (France, Italy, etc..) and then everything in between, it all depends on how well informed and civil a society is. The size of the country too as continental countries are republics are obviously more diverse and complicated by their size and geo-political conditions.