Walmart is also the largest employer in the US and what they pay their employees means that Walmart is about the ONLY place that they can shop. Other companies are using Walmart's "successful" model to get their own pay rates down.
Now, when you factor in the tax breaks that Walmart gets from communities, these same low paid people are paying Walmart's taxes for them. Here's a good case...There is a Walmart in a little town called Roeland Park, KS. There is a super Walmart there. The city currently gets a lot of revenue from the sales taxes at that store.
Walmart is closing that store and moving it to another little town named Mission, Kansas, which is about 1/2 mile from the existing store. One of the enticements to move? Walmart can keep the sales tax for a year or so. Residents in Roeland Park are facing a tax increase to cover the lost revenues from the Walmart...and they'll most likely still shop at the Walmart in Mission, while Mission residents will face a higher tax to subsidize the Walmart.
The new Walmart will be just down the street from a strip mall full of "mom and pop" stores, who have to pay their taxes and are not subsidized at all by the city, which means their costs are slightly higher, and in these days when every penny counts, are at a disadvantage to the megastore that is being subsidized by the city.