Originally Posted by mr ploppy
Amazon lets you do that with ebooks...
The key point here is "lets you." Amazon is not bound by law to do so... they do it purely to placate customers, which in turn is expected to create more business for them. It is a voluntary act, motivated only by the desire for greater profit down the road.
Some of you may have noticed that Amazon has had a habit of canceling the accounts of those who return too much, because they no longer see profit from that account (or suspect they are being intentionally ripped off).