Or they could just not count the purchases beyond 4 CDs in a week, when computing the ratings, while still allowing the customer to buy those CDs.

If what's going on is really what people here seem to think, one might conclude that Amazon is run by total idiots. So maybe it's not what people think? Hard to believe that Amazon foregoes real income this way... On the other hand, I suppose it could be misaligned internal incentives - that it somehow makes sense from some manager's point of view, though not from the stockholder's point of view.