If you have a mixture of products at a price of 4 5 5 5 7 8 and 4 5 is counterfeit 5 is used 6 7 8 is the same product you would find on any merchants shelf for 6 or 7.

If only 4 is returned then on net they made more sales and the consumer sees that he can get product foo on Amazon for only $4!

Amazon makes more money by providing you with a worse experience so why would they improve it?