You're right.
This appears to be the result of having one or two stores (Longos) were they had repeated issues w/infections involving staff.
While possible, based on what we know about transmission of Covid, its highly improbable than the employees contracted this from customers.
More likely they got it at home; followed by from each other at work.
However, we don't know that in these particular cases.
But I understand the problem for Longos, and why they've done this now.
Masks for staff are more important than for customers as transmission liklihood is a function of viral load x duration of exposure so far as we know.
People together several hours per day are more likely a risk to each other.
That said, there is a need to reassure the public and Longos staff; and this may also serve to permit compulsory masks for staff.
I note that I have yet to see that at any of the major supermarkets and in fact, none of the indys I've been in either.