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 raise the price of your dongles to $25, so yours become instantly less competitive.