Earlier quoted context omitted.
> Amazon promoting its own brands isn't much different from a supermarket offering its own brand items. The huge difference here is that when you go into Walmart you don't announce to the company what you want to buy.
Is it really that big of a difference? You still have to go to the one section of walmart where they stock the cheese or car batteries or whatever. Walmart can set up their own shelves to emphasize their own brands if they want. If you pay with a credit card at walmart they can collect the same data about your purchase history as amazon would as well.
The difference, in case it's not clear: Walmart is the manufacturers' primary customer, so it's irrelevant from their point of view what products Walmart chooses to sell in its stores.
The third-party sellers are the manufacturer's customers. The manufacturers still get paid either way. But we're not concerned about the manufacturers; the antitrust issue is with the third party sellers. Since Amazon uses their data to come up with competing products that are sold in the same marketplace, it matters a great deal that Amazon competes with them, and how.