You can't sideload from the Kindle for PC app to a Kindle because books in the Kindle for PC App are DRM'd for that particular computer.
To be honest when people talk about moving books from Calibre to a Kindle they most likely are DRM free eBooks. They either came that way thru purchase or were liberated before being loaded into Calibre.
I sure as heck wouldn't want to go put a new drm'd copy of my eBook into Calibre everytime I switch devices or buy a new model eReader. I have about 1200 eBooks that have been purchased for 3 different kindles, 2 iPads, or a Sony Pocket from numerous different stores. It would be a nightmare to be constantly updating those Calibre files for my current device of choice.