Wal-Mart and obesity

Wal-Mart may attract fat people, but it does not make you fat.

Forbes:

In our first round of statistical analysis we [Art Carden and Charles Courtemanche] found that greater consumer access to a Wal-Mart store was associated with lower body-mass indexes and a lower probability of being obese.

As we gathered more data on Wal-Mart discount stores, Wal-Mart Supercenters, warehouse clubs like Sam's Club, Costco and BJ's Wholesale Club, and other outlets, we found that the correlation holds up under a variety of different circumstances, with a clear relationship between warehouse clubs and better eating habits emerging over time. Further, we found that Wal-Mart's effect on weight is largest for women, the poor, African-Americans and people who live in urban areas.
...
We found the largest and most identifiable link between Wal-Mart's presence in a community and that community's weight is for its discount stores, which carry few groceries. So Wal-Mart's effect on obesity works mostly through an income effect, not a substitution effect. We also found a clear relationship between better eating habits and warehouse club presence, suggesting that people might be taking advantage of bulk buying to stock up on healthier foods.

Our data suggest that we buy healthier food when our purchasing power increases.

So why is Chicago's mayor so opposed to a Wal-Mart in the Southside of Chicago? Who would lose?