I can't speak to amazon specifically, but my understanding of this phenomena in general, especially with ads that are bid for, is that the systems which track and gauge your likelihood to purchase aren't connected to the systems that know you did make a purchase, at least at the point in the system where it's making the decision of what ad to show. It just sees that you've done things that a person who is likely to purchase that thing has done, which, of course you do, because you DID purchase that thing. It's like the ad is chosen based on the strongest additive signal available, and negative signal isn't considered.