They don't claim that at all. Digital items are typically licensed, not owned outright (as is the case with a physical book).
That said, the way the market is currently configured, audio books are a separate product from digital text. That's fine and legal. But, we don't have to like it. I have a legally blind* family member and it always feels a bit like a scam when she has to repurchase books that somebody else in the family already purchased.
* She can see, barely. For complex texts (particularly textbooks), she prefers to have an audio copy as well as the printed/digital text.
You own an MP3 in the same way you own a book. If you email an MP3 to others in exchange for payment, the FBI aren't going to bust down your friends' doors and securely erase the files from their drives, it's up to the licensor to sue you for damages. Likewise, if you copy books and start selling them at the flea market, nobody's going to round up and destroy the unlicensed copies, it's up to the publisher to destroy you in court.
The whole publisher intention behind copyright and license rights is that you shouldn't think of digital products any differently than you think of physical products on a shelf.
In other words, the idea is that just because a digital product costs $0 to copy it doesn't mean it doesn't harm them business wise to make one and not pay them. A physical book/cd doesn't cost as much as its sold either -- it usually costs way way less. So "cost of physical copy" shouldn't be thought as the differentiator...