What if everything you made with the OS was similarly hindered? Launch an auto update that effectively bricks your device unless you agree to the new EULA, blah blah nightmare scenario here.
Less hyperbolically, it's like the first concrete step into killing the freedom to read. My biggest concern with iBooks is whether they have remote wipe and whether you can launch books sans DRM.
It's going to be awkward when the PATRIOTSOPA Act of 2022 remote deletes all of my chemistry textbooks because terrorists can learn about exothermic reactions or revises all of the biology textbooks because evolution has been deemed illegal.
You can play the slippery slope card all you want, and try to apply what we're seeing to all other applications, but I don't see anything to back it up.
Regarding remote wipe. I have no idea if that's in the iBookstore EULA, and haven't found a thing about it. My guess is like apps, you'll have to confirm and download the update yourself. In fact, when apps are taken off the app store, you still get to keep them, whereas Android apps have a kill switch. Have you seen anything that implies remote wipe capabilities?