But what I don't get is why.
I would be uncomfortable with entering a shop without a mask on with no other provision, because I'd feel I was being eyed up by everyone and would be muttered/tutted about and maybe confronted.
If, however, there was a clear process to enter - go to the customer service desk, obtain a lanyard by stating I am exempt, and that there's prominent signage in the store stating that those who are exempt will be wearing such lanyards, and being able to wave it at anyone who says anything - I would feel much more comfortable, because I have proof I am in the right by way of a store issued lanyard and can be told "that's the store's rules, if you dislike it shop elsewhere".
Sorry, as someone with an exemption I simply don't agree - I am avoiding shops where possible (apart from food shops, I have not been in any since the mask mandate came in). I can't really avoid food shops, and I don't want to be singled out by having to go and get a 'note from teacher' to the customer service desk - I just want to go and do the shopping as anyone else would and get out as quickly as possible.