You have to think about the liability for the stores. What if they allow all dogs and one bites someone? What if some of these dogs haven't had all their shots or something? There's no way to check, unless they have someone stand at the door and check vet records that people would be required to bring with them. They can't have dogs peeing and pooping all over, and I sure wouldn't want to step in it while I'm shopping. Of course, you wouldn't want dogs around any food. There's so much to be considered, there's just no way to do it. And could you imagine the people who are not as responsible as others?
It could end up being a nightmare, so I understand why they don't allow dogs in stores.