Yes shops and other establishments can make their own rules up, but as others have already pointed out, many are on their knees and won’t want to risk losing customers by making it mandatory to wear a mask.
On a money podcast last week, they interviewed 2 small businesses, a cafe and a cinema, both in a town in my county.
The cafe owner said that they had only months left to stay afloat (family business, established a long time) if business didn’t pick up from today. She had had one customer by lunchtime on the day of the interview.
She said she couldn’t wait to have more customers in when social distancing rules changed, and to be able to chat to them properly without masks in the way.
I can’t see her refusing someone entry and am sure many other business owners will feel the same.
Op, you are getting a hard time here, but then MN is a weird place. There are many people who will stop wearing masks (myself included) and that is despite me having a DC working in retail. She is choosing to carry on wearing them but has been told it is not mandatory (major supermarket chain) so I can’t see her employer stopping customers without them.