Amazon's new order virus

After making a purchase at Amazon.com, I noticed that there was a form box on the order confirmation page where you could enter the email addresses of your friends/family and invite them to save 10% on the same thing you just bought. If they go to the site and buy your recommended item, not only do they get a discount, you get an Amazon store credit for the dollar value of their 10% discount.
Amazon doesn't tout this as a viral marketing ploy, but that's just what it is, and you don't have to be Amazon.com to benefit from that kind of an offer. Any online store could do the same. I don't know if any firm data exists on how many of the average online business's customers join the affiliate program-I would think it's rather small. I know a lot of people who give me a blank look when I mention affiliate programs. But this little twist makes it a point of purchase whim-why NOT send a quick email off to your friends saying what you bought and giving them a store discount? That's what separates a really good viral idea apart from your garden-variety email chain letter: if the sender feels like they get a benefit out of sending it on and the recipient is glad you sent it, how can you lose?