It's not just the books, but the reader.

This is straight Stallman: if you don't fundamentally control your computing environment, you don't own it, and you have no freedoms within it save those granted by the entity that does control that environment.

A Lessig might modify that slightly and say that your freedoms are actually balanced between four influences: Law, Norms, Market, Architecture. In the case of DRMd works on proprietary hardware, law and architecture are both opposed to the rights of the individual. Presumably Amazon responded to social norms and market pressures.

I prefer having more power on my side. It's a key reason I've avoided the e-reader game to date, despite some interest in going there.