I don't know about the legality, but this is seems like a very useful feature.

When I was a teen (well, younger teen), I learned to understand and comprehend western accents by watching a lot of videos by North American or European creators and using close captions on YouTube. At this point, I don't need them anymore, but I certainly did when I first got started.

Clearly this feature on audible can serve a similar purpose, and can definitely help students like myself in developing countries who are taught how to read and write English, but not exactly speak it - at least not in the same way as the western world.