I heard that this was the reason they were doing it too. The Government is panicking because not enough people are going to the shops. Like you, I think it could have the opposite effect and discourage people from shopping on spec.
Certainly it seems odd from a health perspective to be making masks mandatory now given the low incidence of the virus at the moment and when it was rife back in March and April we didn’t have to wear them. Indeed at the time the stated belief was that masks might make people complacent and not socially distance.
A friend of mine who works in an off licence texted me today. She was not happy not only because she would have to wear a mask all day herself but also the issue of enforcing compliance from some of her customers. Some of the regulars are interesting characters to say the least.