If your store closed for a few hours would that really be a disaster? Would you manager actually fire you if you refuse to work that much overtime? You're not obligated to work yourself into a mental breakdown.
And that the business is still making money to pay the employees. A lot of restaurants are saddled with debt and can't just pay employees when they aren't generating work.
For a store that's true, but it's also happening in residential care facilities where it can be a matter of neglecting care or even life or death when they are severely under-staffed, and the staff who do come in have more work, longer hours, and more patients per nurse than they can handle. Should an infected but asymptomatic person come to work and risk spreading it to their patients? Is that worse than the patient getting neglected and at risk or dying because of a staffing shortage? I don't know the answer.
A surprising number of stores (and restaurants - maybe especially restaurants) operate on razor thin margins.
Plus contrary to some romanticized images of the successful small business owner driving Porsches, most store/restaurant owners probably barely make enough to make ends meet.