[DiapDealer]

It's not a conversion. It's just a file rename. Amazon's AZW file extension doesn't really tell you much about what's under the hood. It could be a standalone KF8 file, could be a plain-jane mobi, could even be topaz book. That particular file extension (AZW) has very little to do with the book's "format."

As for why it's being renamed to AZW3 upon import: that's not calibre doing that. That's your third-party plugin.

If the actual underlying format were a regular old-fashioned mobi, it would import as *.mobi file. If it's a standalone KF8 formatted file, it will import as an AZW3 file (a topaz file would import as an HTMLZ format). It's just an attempt to make the file extension actually MEAN something with regard to the book's underlying format. For all intents and purposes, AZW3 has come to be identified as a stand-alone KF8 formatted ebook.

It may not work for everybody, but it works for the vast majority.

Besides, I don't think you can't really sideload KF8 books into the iOS Kindle app regardless of the file's extension. That's why the whole AZK preview format came into being.