Also, the same-product-again recommendation doesn't have to be a great one; it only has to beat the alternative it displaces. So I could find it plausible that taking a swing on you buying another of a product you already bought beats out whatever the 5th-best recommendation happens to be.

Given how widely this policy is criticized, Amazon must be very aware of it and they must continue to do it intentionally. I would trust, given the amount of money on the line, that they see evidence that tells them it's a reasonable choice.