Though I consider the specific situation with iBooks and DRM to be both mild and reasonable, it does invoke a thorny legal and moral question: exactly how much of your rights are you allowed to sign away voluntarily?
Extreme ends of the spectrum are very clear: selling yourself into indentured servitude is a violation of basic human rights, while selling your time for eight hours a day is not.
Many advocates for digital freedom would argue that allowing customers (particularly the non-tech-savvy who don't know what they're agreeing to) to give up the rights to their devices and content is inherently immoral. I don't agree entirely, but I see their point, and if there was a ballot initiative proposing consumer protections against things like DRM lock-in, I would likely vote for it.