You just described my sister..... She doesn't even think her '09 Versa has a rear defroster...I agree with everything you've stated.
The reason the AC kicks on when the windshield vents are chosen is because of idiots that refuse to learn how to operate their HVAC system. When their windshield fogs up, they push the windshield shaped button. If the AC didn't also automatically come on at the same time, they would die of a wreck before they figured out why the AC compressor needs to be on to dry the air and remove the fog from the inside of the windshield.
Basically, it is a safety feature to protect functioning illiterates that somehow got a drivers license.