Originally Posted by rstoothoff
I had assumed that Calibre was designed to be used to convert from .epub to .mobi not only for "personal use" but also for submission to KDP.
Conversion is for personal use. People can use the output for comercial purposes, there is nothing that prevents this. Many people do upload to services like KDP but that isn't the purpose of any conversion output. It has been and still is designed about reading books you own on devices you own.
One big reason for this stance (especially with a format like MOBI) is it's impossible to produce a 100% compliant file when dealing with a proprietary format. All information about MOBI has been reverse engineered and there is no way to know if something is missed. There is also the issue that Amazon can had has changed the format for no apparent reason other than to break tools like calibre.
Ultimately Amazon owns the publishing service, the format, the production tools, and the devices/software that reads the format. They control every aspect of the process and can require any arbitrary rules they want.