A mask is only really useful if you're infected and you wear it to contain the virus and not cough it all over people (you would need to have washed your hands etc after touching your face when you put it on and all that).
The people walking around with masks on should also be wearing eye protection. If someone coughs or sneezes on you, the virus will enter through your eyes even if you were a mask.
Supermarkets would need to provide masks and eyewear, and since the nhs is struggling to get enough, why do you think supermarkets would be able to get enough for every customer? And do they know how yo avoid cross contamination when handing them out? Do they know how to remove and dispose of them? No.