It's because Walmart provides goods at low cost to the masses--and Liberals hate the masses. Daniel Greenfield explains:
Why does the left really hate Walmart? It doesn't really have a lot to do with unions and has a lot to do with class. Walmart's crime is industrial. It's the crime of the factory and the supermarket and every means of mass production and consumption. It makes cheap products too readily available to the masses. Liberals like to believe that they oppose consumerism, but what they really want to do is raise the entry levels to the lifestyle. Liberal consumerism is all about upselling ethics.
When tangible goods become too easy to produce, you add value through intangibles. The fair trade food tastes the same as non-fair trade food. Organic, a category with a debatable meaning, doesn't really provide that much more value. And environmental labels are worth very little. And yet the average product at Whole Foods is covered in so many "ethical liberal" labels that it's hard to figure out what it even is.
Intangible value is all about class. And class is all about creating barriers to entry.
Liberalism has become a revolt against the middle class that its grandparents struggled to reach, a rejection of their "materialism" while substituting the "ethical materialism" of liberalism in its place that envisions a much smaller upper and middle class that derives its wealth and power not from hard work in the private sector, but highly profitable social justice volunteerism in the public sector.
An American Dream of universal prosperity has been pitted against the left's dream of a benevolent feudal system in which the few will be very well paid to oversee the income equality of the many.Hence, Naomi Klein's anger that consumer goods have been made available to developing countries.