Are you talking about low quality or counterfeit? Personally I don't care about counterfeit, but I do care about quality. In any case, Amazon already has been tackling it to some extent.
http://www.businessinsider.com/how-amazon-fights-counterfeit...
How you do that is part of the lifecycle of an Ad platform. The ephemeral "quality of ad" is completely subjective and will forever be inadequately measured (to anyone's satisfaction for very long). Eventually all ad networks are subject to market pressures in pursuit of margins. Third party ads eventually arise from the existing technical implementation of ads (tons of untrusted code running in js and vpaid applications), even when "prohibited". There's nothing special about Amazon's platform. Lots of platforms have been the "new hot thing" with "total control".
The chances are users will be happier with a product through natural selection ( eg. better/trustwurthy reviews), than products through ads.
Although, Amazon could earn more in short term, users are less satisfied and they lose money long term.
What happens if Chinese vendors buy all the ads and set a lower price ( which they can). Then most buyers won't be satisfied on Amazon ( terrible English) and they start using AliExpress ( cheaper and same shipping terms after eg. 1 year of Chinese vendors that dominate Amazon). This "free money" could be their downfall.
Already technical users here on HN don't want to use Amazon for the risk of receiving counterfeit products.