Amazon is a marketplace/platform more than it is a direct seller at this point. The ads they're showing you were likely purchased by sellers, possibly including other sellers of the product you purchased (if you were shopping around).

Depends. They might want to to get you to come back and write a review/buy additional stuff.

On the other hand, this could be an affiliate that tries to generate any click so he gets a commission on your further purchases.

Whether it's Amazon wanting a customer to take additional action or an affiliate trying to drive traffic, relevant retargeting is much more effective than non-relevant.

And from the affiliate's perspective, interfacing to good retargeting is just as easy as interfacing to bad. That is, it's a black box owned and managed by Amazon.