But nor are shops required
not to enforce it - they have the right to set their own rules for who they will admit into their premises. That's not about enforcement of the law, as staff at a shop are not empowered to enforce this particular law; it is a shop enforcing a standard on its customers. And if, as
@Essan suggests is the case, many of those who refuse to wear are neither exempt, nor engaging in protest, but simply being awkward, then it's quite likely that those are customers that store doesn't want anyway.
As for the question of protest, it would be nice if some of those objecting to masks made more of an effort to be civil in their disagreement, rather than continuing to deride anyone who dares disagree with them.