[tomsem]

AZW3 is explicitly for side-loading, e.g. with Download for Transfer via USB from your Amazon account. Amazon never explicitly supported it for Send To Kindle (except as was pointed out when you were able to fool Send To Kindle by renaming with .txt extension, and just as .epub was supported by using .zip extension).

Dual format Mobi was fully supported for Send To Kindle, until it wasn't, and that is what most third party content providers supplied for Kindle users (some, like Gutenberg, still do). It has all the same features as AZW3.

It's true that AZW3 has been reverse engineered so other eReaders can read it directly, and it can be generated somehow by the likes of SB (using calibre apparently).

But SB knows very well that AZW3 is incompatible with Send To Kindle, that it doesn't support advanced typography features, and that Kindle customers cannot add these to their Amazon cloud storage as is.

Instead they discourage Kindle people from even trying the ePub (which they explicitly say is for 'All devices and apps except Kindles and Kobos') and make it sound like AZW3 is going to give the best results, when functionally that is just not the case. You need an epub file to get the best functional results, via Send To Kindle.

It is not rocket science to do some minimal testing with an actual Kindle and Kindle Previewer and Send To Kindle as a sanity check. Even running it through ePubcheck would probably suffice. I assume their CSS and HTML markup is consistent for all titles so they only need to do this once.

If they ever want donations from Kindle users they would make sure the ePub works okay and announce 'we have made sure our ePubs convert well with Send to Kindle' and get rid of the AZW3 option altogether.

It just seems lazy and short-sighted.