Nothing, and that's a good thing, because now that wearing masks is a partisan political issue, grocery stores should feel entirely within their rights to ban people who refuse to wear masks even if they claim political grounds, and they should not be forced by the government to let people in.

In America, the default is that private businesses, even those who provide a service to the public, get to choose who they engage in business with and how, with no interference from the government. There are very narrow exceptions (Civil Rights Act, ADA, antitrust, etc.) when absolutely necessary.