Shop workers are at a high level of risk. Why wouldn't you want to do anything you could to reduce that risk?
How many people have a genuine medical reason for not wearing a mask? And is that reason more severe than the effects of COVID-19? I have an ileostomy and I remember a fuss in the "ostomy" community when seat belts were made compulsory. Didn't we have a reason not to wear one? As the belt came across the body, might it not crush the stoma? This went on until the Ileostomy Association pointed out that a damaged stoma was nothing compared to going through a windscreen and fracturing your skull on the road. That ended the debate.
Of course, in that case, the greater danger was to ourselves. I can't help wondering if some people are reluctant to wear masks because the aim is to protect other people, rather than themselves. I think this is why many Americans struggle with it. They don't accept the idea of making sacrifices for others, hence their resistance to a tax-funded health care system.
In the USA, the virus has been heavily politicised, with resistance to the lockdown, mask-wearing, etc vociferously opposed by right wing Republicans. Apparently some Tories are cutting up their party membership cards in response to the face mask announcement. Let's hope that common sense prevails.