There is a big difference between music and books. Music is intended to be listened to over and over. Books are not expected to be read more than once: certainly not over and over constantly.

Also, he's not buying it at all. He's borrowing it from the library, which is obviously okay with publishers, since they haven't gone after libraries for lending physical books. (Well, they have, on occasion, but not with any real force or success.)

In addition, it is not up to producers to define theft.

I'm sure that the library is following rules set up by the publisher's equivalent of the RIAA, probably the AAP. Publishers have had an adversarial relationship with libraries since book publishing became big business. Until recently, it wasn't a huge deal, as libraries still had to purchase all of the books. It was little different than a used book store. But now they can have unlimited copies.

The thing that would most accurately match how physical libraries work is if the DRM worked like a semaphore and the library had to pay for each license. Then copying the audiobook would be equivalent to photocopying the whole book. This may be the way it works for the library, only they keep track of the number of licenses, rather than there being some technological solution.

Does your library require that you relinquish the audiobook somehow? It may not make any difference; they might just assume you keep the license for the whole two weeks and not bother worrying about early returns. I'd say there's a mild ethical problem with doing what you're saying, if you're checking out their license, un-DRM-ing it, and then immediately relinquishing the license. That means that a nearly unlimited number of people can "read" the book. However, if you let the two weeks run out, then I don't see much of a problem. You are still preventing someone else from obtaining that license during the period of time that you would normally be reading it, and the publishers are okay with that. I'd say that as long as your actual "reading" time (and I'm not counting time you're not listening to it) doesn't exceed the two weeks, then there's no ethical problem; you're just time-shifting for convenience.