Captions is exactly what's being offered though. Amazon is not providing an ebook, they're providing captions in-sync with the audiobook.
The difference between an audiobook and a video is pretty small in a significant portion of YouTube content.
The difference between an audiobook and a video is pretty small in a significant portion of YouTube content.
As the GP said, there's quite a few youtube videos with a completely static background where the content is the speaker's words.
AFAIK, they would have to obtain prior permission from the copyright owner before re-distributing a modified version of the copyrighted work.