These aren't visual watermarks like what you'd get on physical paper. They just alter the digital copy so that it can be traced back to the buyer, and people a

These aren't visual watermarks like what you'd get on physical paper. They just alter the digital copy so that it can be traced back to the buyer, and people are deterred from sharing it on the web. It's a very elegant solution if the 'watermark' can be made hard enough to remove, and I can see quite a few workable approaches.