I’m sorry, but that makes a complete nonsense of the whole thing. If it’s for safety, the people who are in the shops most need masks most. So either they are all talking rubbish, or the masks are to make people feel safer, or it’s about continuing to control the population. Or a combination.
It is a tacit admission that masks are no longer about safety (if they ever were). They are likely to prevent a handful of infections at most given how poorly most people wear them. They will probably cause many more infections than they prevent because of the unsanitary manner in which most people are forced to use them, having no proper means to dispose of single use ones, or to sanitarily apply and remove reusable ones.
Masks may have had some value earlier on in the pandemic but they are no more than a gesture intended to allay the fears of vulnerable people and the anger of locktivists. They will be effective in neither regard, because vulnerable people will associate a mask-compulsory supermarket as dangerous, and locktivists won't be happy until we're all chained up in our beds with masks glued to our faces.
The whole purpose of this new ruling is to pander to public opinion. We have been going to supermarkets throughout this pandemic, earlier in which cases were much more prevalent. If you are sure that wearing a mask in a shop reduces the spread of the virus please post a link to one case / story - just one will be enough - where a shop in this country has been identified as a source of a covid outbreak.
There can be no doubt that supermarkets and shops will have been the source of some outbreaks, or at least individual cases. They were pretty much the only place most people were near other households during the end of March and beginning of April, and yet cases were still increasing (and many more infections still happening).
But they are now just a minority of the places where people could be infected, and the absolute number of cases means that mandating masks is simply a waste of time.