I think grocery stores are in a tough position. People need food to live and it's hard to turn someone away who's trying to buy food in order to live. Some people don't care to wear masks, so those people obviously suck, but others cannot get their hands on any, as there is just not enough supply to go around. In fact, people w/o internet (e.g., 80 y/o elderly people) are having an especially difficult time with this. A better thing to do is sell people bandanas at the front door and say, "either wear a mask or buy a bandana for $3." ($1 a bandana would be selling them at cost, but $3 would be enough of an inconvenience. If someone is paying $3 for a bandana, they will use it or they'll be wasting $3.) At least that's what I would do personally. Would most likely solve the problem.