[jackie_w]

To add some more (completely unfounded) speculation ... the presence or absence of hyphenation css in the azw3 itself may not be the controlling factor. To use the Kobo as an example, it has 2 main book rendering apps one for standard epub and one for kepubs. Each app has its own, but different, overriding css file built into the firmware. It's how the customisation of font, size, line-spacing, margins, font weight is handled. Maybe the hyphenation css will be placed in a similar Kindle css override file.

In my own experience these override css files can be a boon or a curse depending on your point of view and how aggressively they attempt to change each book's own css file.