There is no reason to close the store. It is very unlikely that any customers became infected by the workers. I am gonna assume the health police have been unable to track the grocery workers as a source for infections of customers.
It is not a matter of the store being more hazardous. It is a matter of a store of that size has a lot of workers a certain percentage of whom will test positive. As would probably a certain percentage of shoppers.
Grocery stores take multiple precautions to prevent contamination. They are also an essential service. I would consider them low risk environments as far as getting the virus from the store or workers. The risk would be from getting it from other customers. But that risk must be weighed against having an essential service available to those who depend on its products.
A better idea would be to limit occupancy and to offer special shopping hours for certain at risk persons.
I agree with most of your post, but it must be pointed out, that the virus lives on surfaces more than long enough to be passed from one person to another by touching the same product, and one of the reasons I stopped shopping Wal-Mart is precisely the obtuse carelessness with which goons handle the merch, and how little time Wal-Mart directs their employees to spend cleaning/refacing product -- exactly the lack of attention to detail which doomed KMart.