Walmart's business wasn't built on selling to low income people. It is built on selling to everyone in places like Fort Stockton, Texas. The person renting a dilapidated trailer and the rancher with 150,000 acres of cattle and oil leases both shop there because there aren't a lot of other practical options short of driving to El Paso. Not saying that there's not a significant difference in branding, but a lot of that difference is that Whole Foods brand images for people who are offended by Pop Tarts and HFCS on "their" store's shelves. On the other hand, Walmart's potential customer base is anyone who is not offended by shopping at Walmart for the vastest majority of grocery purchases.