The point is that a lot of companies used to treat their employees like Costco
True.
but the American people sold them out to save a nickel at Walmart, who treats employees and suppliers like shit.
Partly true. We have the Sherman Act and Clayton Act, already on the books, and have for over a century, that should protect suppliers against the downstream effects of private monopolization/monopsony. The courts - these are mostly Republican appointee judges - unfortunately gave up on enforcing those laws as written and replaced it with bespoke/pro-business interpretations of said laws (for an example of this in action, the Supreme Court's Leegin Creative case is Justice Kennedy at his worst). The average consumer has no way of learning about supplier relationships, so it seems a bit silly to rely on consumer purchasing decisions to sort out policy preferences as opposed to heavier regulation of the business to business marketplace. I don't think Wal-Mart becomes Wal-Mart (or Amazon, Amazon, etc.) if Warren Court descendants stocked SCOTUS and the COAs.
Acceptance of treating employees like shit is a downstream effect of a whole bunch of commonly held but toxic beliefs about labor in general, both from the right (hostility to labor unions, a presumption that certain jobs need not provide a living wage as a matter of policy), and too many people on the left (for example, the caustic attitude of otherwise well-meaning educators towards work that doesn't require a college degree). Because everyone benefits from cheap labor, no one really wants to do much about it. The Wal-Mart/fast food situation is the social problem unions and labor regs were designed to fix but lots of people think that's sOcIaLiSm instead of leveling the bargaining table and labor marketplace.