The conpanies you listed all exert their own monopoly powers. Ask anyone supplying those companies about what extra capital they need to raise to meet the company's requirements. Walmart makes you buy into their RFID system. That is called a barrier to entry.
Walmart is also thought to engage in monopsony, which is requiring sellers to sell to them at disadvantageous prices.
I’m skeptical of that claim, but it would offset collusive pricing from suppliers.
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