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.
More specifically, the responsibility for chasing down the manufacturer for a refund is shifted onto the retailer, not the consumer.
The problem with being an "everything store" is that everything also includes an overwhelming majority of crap, looking at you crappy "genuine sony" camera batteries that held about 10 minutes of charge, fulfilled by amazon.