Earlier quoted context omitted.
Because it can create any product that is selling well, and at any price point, because it _also_ has the right to subsequently raise _your_ price. You're selling dongles for $20. You sign a contract with Amazon. It realizes that dongles are lucrative, and starts getting them manufactured. Hey, it can sell dongles for $20, too. But then they're competing with you, because you both sell $20 dongles. So, solution, they…
Substitute Walmart for Amazon. Substitute corn-flakes for dongles. Substitute Kelloggs for "me". That is the situation right now at Walmart. Kellogg's Corn Flakes are $3.28 for an 18oz pack. Great Value (Walmart own-brand) Corn Flakes are $1.43 for the same size package.
It’s complicated for Walmart, because there is documented evidence of them telling suppliers what wholesale price they will _accept_, but that does not change that it is ultimately Kellogg’s wholesale price that is being met by Walmart.