Once again big media companies are just making up new rights for themselves. The idea that the properly licensed spoken word version of a published work does not intrinsically include the text is laughable. What's next? Kindle books are infringing on copyright because they reformat the text of the book into a version that fits the user's screen size and preferred font? I mean why not?

And I suppose YouTube is infringing on copyrighted videos when it adds machine-translated CCs, right?

Modern copyright is indistinguishable from rent-seeking.

I don’t understand why publishers are against Audible Captions. That feature makes audiobooks less pirate-able, and it also won’t lead to more pirating of ebooks, since it’s better to share a cracked ePub rather than ill-formatted captions. The only revenue loss I can see is from customers who would otherwise buy both the Kindle and the Audible version, but that must be a small fraction.

My guess is that they just want to be paid more for them.

(i.e. They sold the rights to sell the audiobook alone; if audible wants to also include a text version of the book, they want more money).

At the expense of accessibility.

Audiobooks accessible to the deaf and hard of hearing.

They're not against it. They're against licensing it for free, and what permitting media-shifting surprises might do to damage all their licensing agreements.

Well, technically it's a derivative work, they have every right to sue.

If the translation is performed on device then Audible would not be liable, considering non-infringing use (see: VCR).