It sounds like they are checking to see if the DRM is functional. The app stops working if it isn't. I can't say I'm a fan of DRM but this really doesn't sound like a case of "stop the jailbreakers."
Yeah, that's what I was trying to say.
I kind of find this reasoning likely. Why else would they go to this much trouble to annoy jailbreakers? (In iBooks, of all places!)