Of course they should be unionized, when unions were big, the middle class was also big, when the unions started declining so did the middle class. Coincidence? I think not.
The haggardness of poverty is everywhere seen contrasted with the sleekness of wealth, the exhorted labor of some compensating for the idleness of others, wretched hovels by the side of stately colonnades, the rags of indigence blended with the ensigns of opulence; in a word, the most useless profusion in the midst of the most urgent wants.Jean-Baptiste Say
Well, seeing as Walmart is neither of these things, not even close, and is just a simple retailer with tons of competition in the form of other simple retailers, "the effects of market power" are negligible.
Now let's move on to "labor market coercion".
Hm... according to Wikipedia, economic coercion requires market power, so it too can be disregarded.
Now, what are these "severe inequalities" you speak of?