Common sense. Amazon want to ‘own’ the customer relationship and go to great lengths to that end then it follows they will have to ‘own’ any liability too. Imagine you went into a retail store with a faulty product and they were to try fob you off saying you need to go to their supplier/wholesaler/manufacturer. It mostly doesn’t work that way thanks to most consumer protection laws in place.
I understand where you are coming from, but there are lots of consignment stores and flea markets, at least in the middle parts of the US. The person you pay at the front counter simply records the transaction for the merchant. Amazon is a sort of cross between traditional retail and a consignment store, so it's not obvious (or "common sense") where product liability should lie.