The usage pattern of a book, either physical or audio, far more closely matches that of a movie or video game than a piece of music.
A statement strengthening the argument that stripping the DRM from an audiobook is an act of piracy.

(Keep in mind that through all of this, I'm just playing devil's advocate here. I personally wouldn't think twice before stripping the DRM from any audio file that I'd obtained legally: it's the only way I could play the file on my chosen playback device. It's just fun to have discussions about ethics on this BBS, everyone makes such great arguments and gives such great examples.)