It comes from "Digital Rights Management" basically it's program designed to pee legimite customers in the eye (and thats who they hamper, since pirates bypass them anyway). It's sort of like saying, we think you are a thief who didn't pay for this, so we are going keep an eye on you. I don't know how agressive the DRM generally is on ebooks, but it can be pretty bad on software (limiting number of instals for example. "Oh, you already installed it three times? And you needed to reinstall your computer? Too bad. Now go buy a new one." ).