The add on links away from the legal product and onto a pirated download for the same product. So, when you want to buy something, as long as it's available in digital media, you'll get the illegal copy instead.
To Amazon, this is the equivalent of a junkie sitting outside your shop doors and promising to nick anything you want without the shop owner even being aware of it.
Given the time of year as well, this will cause havoc to the online retail store; and is sure to result in countermeasures that can only start at DRM's.
The pirates themselves, though devoid of eye-patches, are using the blend of retailing and torrent tracking to 'redistribute the wealth', whilst getting a healthy link to themselves.
Given the amount of money at stake in this game, one can almost imagine the Amazonian Armada sailing up behind the HMS Firefox as we speak.