<blunt mode>
@user743
If you wish to use Calibre's Convert: Live with it or don't use it. That is the way it works. Convert is not broken, it just does not meet YOUR standards (you are not alone. Others refuse to use
Convert)
If you want to use the Editor to
create a AZW3: jump through the hoops as outlined by @eschwartz. 'Thems your options today
</blunt mode>
It is the final results that count , not the elegance of the code behind the display.
I don't have a clue on how many Calibre Convert users NEVER look at the resulting code (clearly, I am not one of those

) behind their books, but I would guess from the questions: >30% (and how many never made it here to MR?).
Convert has to deal with many code and reformatting issues, turning off class="calibre#" generation just is not worth the headaches/bug reports that would create. Note: in a few cases,: Calibre convert modifies valid code to provides alternate code WORKAROUNDS for devices with poor render engines.