Amazon supports .mobi because if they did not, they would be sued by everyone who bought .mobi books for use by MobiPocket at
http://www.mobipocket.com/ many years ago, including me. I read them on my PalmOS devices, plus my Win98/Me PCs.
Amazon bought MobiPocket, and also the .mobi format, so they would have little competition when they got into selling ebooks. Amazon did everything it could get away with to kill off .mobi in favor of its own formats.
The Federal Trade Commission would have viewed their actions as anti-competitive and on the road towards monopolistic if they had dropped support for .mobi.
Personally, I converted my .mobi books to epubs (via Calibre) years ago.
DaltonST