True. But in this case, I think the two happen to synch up. Do you think it would be ethical to photocopy (or, to remove the expense of the copies from the equation, scan-in) an entire book that you borrowed from the library?

How about a different metaphor? Let's say you rent a video game or a movie from someplace that rents them, and you get around the DRM to make a copy of your own, to enjoy on your own time. One argument says you're just time-shifting, another says that you're committing an act of piracy.

An audio book is more like a music album than any of those things above, and I think it's still unethical to remove the DRM from recordings that were published that way.

Of course, I also think it's pretty unethical to publish DRM-protected audio files in the first place: It unnecessarily ties the file to a specific playback device (or a specific subset of playback devices). But once a publisher has chosen to publish it that way, your ethics are bound by their intentions.

Damn, did I just re-enact an XKCD?