This can't be a violation of any law or ethical rules... Does anyone seriously think serving only good and relevant ads is an ethical obligation. Can't retailers accept ads from anyone without vetting for quality control.
The only thing I can think of here is FTC is trying to make a weak case that Amazon is a monopoly by claiming look they had the power to show crappy ads. Which is a weak argument.
Maybe Amazon is too big and too dominant in the market. I don't know. But this article and this aspect of the case looks weak.