Doug Henwood continues the discussion, making the cogent point that while Walmart wages may be average for retail, that Walmart has played a major role in lowering wages and cutting benefits. Because of their size, they set wages as well as prices.
By lowering the cost of the bare minimum, Walmart makes it a lot easier for all employers to pay less. That brings a smile to the faces of stockholders, of course, but no so much the average worker.
Walmart is the 800 lb. gorilla of retail, so they get the focus. If Walmart raises wages then the prices they charge will also increase, something which will hurt their low-income shoppers. If your family earns $25,000 a year, then saving $20 a week on food by shopping at Walmart is not trivial.
Walmart could raise wages and not raise prices, cutting their profit margins. However, this could well cause shareholder lawsuits.The underlying problem is that we have a predatory economic system based solely on maximizing profits which gives no thought to the health of the system overall.