Walmart has a 25 percent share of the grocery market; back in the 1940s, A&P never got above a 12 percent share because of an antitrust case. But there's no serious threat of an antitrust case against Walmart.

Walmart talks about buying local, but that mostly means buying from big producers in the same state, not buying from small farmers in the immediate community. And it's not just Walmart squeezing those small producers:

Four meatpackers slaughter 85 percent of the nation's beef. One dairy company handles 40 percent of our milk, including 70 percent of the milk produced in New England. With fewer buyers, farmers are struggling to get a fair price. Between 1995 and 2009, farmers saw their share of each consumer dollar spent on beef fall from 59 to 42 cents. Their cut of the consumer milk dollar likewise fell from 44 to 36 cents. For pork, it fell from 45 to 25 cents and, for apples, from 29 to 19 cents.

Walmart doesn't bring new jobs with it, either, not even crappy Walmart jobs. Instead, it brings unemployment and poverty:

A study published in 2008 in the Journal of Urban Economics examined about 3,000 Walmart store openings nationally and found that each store caused a net decline of about 150 jobs (as competing retailers downsized and closed) and lowered total wages paid to retail workers. [...] These shifts may explain the findings of another study, published in Social Science Quarterly in 2006, which cut straight to the bottom line: neighborhoods where Walmart opens end up with higher poverty rates and more food-stamp usage than places where the retailer does not expand. Other studies have found similar results, with Walmart workers earning less than other retail workers and Walmart pushing down average retail wages and health coverage rates.

Walmart doesn't just hurt the people who work in its stores and supply chain. It and a few other companies are consolidating control of the entire American food chain, hurting small farmers and food producers, other grocery stores and their workers, consumers, and local economies. Stacy Mitchell of the Institute for Local Self-Reliance shows how Walmart's death grip on groceries expands well beyond the people who actually go into the stores to work or shop: