I guess I'll wade in here with a perspective. I'm someone who has been violently opposed to any DRM in my music files, and never bought a single track from iTunes in my life. I'm also a Kindle owner who happily buys DRM'd books from Amazon all the time. How can this be?
The difference I guess is how I want to consume the two different types of media. I want to be able to play my music again and again, now, and 20 years from now, in my car, on my media player, on my 4 different PCs, and on my living room stereo. DRM basically makes this impossible, or so convoluted as to be impractical.
OTOH, I only want to read a book once. The only place that I want to read my eBook is on my Kindle. I buy a Kindle book, I read it on the Kindle, and I'm done with it. That fact that it's DRM'd never affects me. I don't care that I can't loan it or resell it later, these are just not big concerns for me. I'm willing to give up those things in exchange for the convenience of a lightweight electronic reader.
I'm also aware that Amazon has no choice, just as Apple had no choice when they first introduced iTunes. The DRM requirement is being driven by the publishers. If Amazon wants to get the big publishers on board today, there must be some kind of copy protection in place to satisfy the dinosaurs. Over time, I suspect this will change, just as it did with iTunes.