Some newer print replica eTextbooks sold on Amazon are available on Kindle for PC/Mac, but only when running the latest version (1.26). The examples I have seen so far are all expensive textbooks from Pearson Education, such as
this one and
this one.
The book descriptions have a note "This title is not supported on Kindle E-readers. On PC & Mac, the latest version of the Kindle app is required." There is also a popup that states: "This title is only compatible with Kindle for PC & Mac (versions 1.26 or later) and is not supported on Kindle for Windows 8 app. To read this title on Windows 7, 8, or 10, you can use the Kindle for PC app."
Similar, older titles from the same publisher are compatible with older versions of Kindle for PC/Mac. I speculate that the restriction of the newer books to the most recent Kindle for PC/Mac is due to these books being made available only in KFX format.
There is precedent for books being made available only in KFX. That is currently the case for books in Indian languages, Arabic, and Traditional Chinese. It appears that Amazon had to make these KFX-only because the KF8 renderer has issues with these languages.
There is no obvious enhanced capability apparent in these textbooks from their descriptions so I doubt that KF8 limitations is the reason there.
I speculate that this is being done specifically to thwart piracy. These are very expensive books that are sold in limited quantities. I suspect that it was the publisher's decision to opt for the new, stronger DRM of KFX, despite it resulting in limits placed on the book's availability.
This might be the start of a new trend. I will be on the lookout for more KFX-only books appearing on Amazon.