If Walmart took responsibility of the animals in it's care I'd have no issue with them selling pets. However it sounds as if they are negligent and only sell pets to make a quick buck.
I disagree with the notion that mom and pop stores are better than larger chains and franchises. In New Zealand the bigger pet stores have staff who are passionate about animals and their care and spend a lot of time interacting with the animals and making sure they have everything they need. Including walks and exercise outside of the store. The bigger chains seem to be more careful about who they hire. Some of the mom and pop stores here are also very untidy and often the puppies are sitting in their own excriment which obviously hasn't been changed for hours.
For me its not so much of a case of where animals are sold but what care is taken to provide the animals a healthy, safe and enriching environment.