What Walmart does is create a local
monopsony (not monopoly), i.e., they are the only employer in some areas, and hence they can pay as low of wages as they want because they don't have competitors.
However, a really smart guy, Obama's director of economic policy has produced research demonstrating that Walmart actually helps the poor by providing low-cost goods.
http://www.americanprogress.org/kf/w...rogressive.pdf
I haven't read the paper but it
does sort out both effects (low wages/low prices).