It's about access. If the poor and rural folks don't have access to supermarkets because there aren't any in their area...but there are plenty of fast food chains...then they don't really have a choice, do they?
• Low-income zip codes have 25 percent fewer chain supermarkets and 1.3 times as many convenience stores compared to middle-income zip codes. Predominantly black zip codes have about half the number of chain supermarkets compared to predominantly white zip codes, and predominantly Latino areas have only a third as many.46
• Low-income neighborhoods have half as many supermarkets as the wealthiest neighborhoods and four times as many smaller grocery stores, according to an assessment of 685 urban and rural census tracts in three states. The same study found four times as many supermarkets in predominantly white neighborhoods compared to predominantly black ones.38
• Another multistate study found that eight percent of African Americans live in a tract with a supermarket compared to 31 percent of whites.42
•Among affluent neighborhoods in Atlanta, those that are predominantly white have better grocery store access than those that are predominantly black, indicating that race may be a factor independent of income.30
• In West Louisville, Kentucky, a low-income African American community that suffers from high rates of diabetes, there is one supermarket for every 25,000 residents, compared to the county average of one
supermarket for every 12,500 residents.17
• In Washington, DC, the city’s lowest income wards (Wards 7 and 8) have one supermarket for every 70,000 people while two of the three highest-income wards (Wards 2 and 3) have one for every 11,881 people.20 One in five of the city’s food stamp recipients lives in a neighborhood without a grocery store.37
http://thefoodtrust.org/uploads/media_items/grocerygap.original.pdf