Look at the shape of the rear doors. The rear windows are taller than the front ones due to the raised roof. Not a child safety feature, just lack of space for the window to retract. Also take note that the skin on the outside of the door is longer than the inside of the door.
Take a look at the rear doors on my previous 2000 Nissan Crew Cab:
The windows would roll down fully. But the doors were a different shape. Even that truck had the LATCH safety points, mechanical door safety locks, and electric window lockouts... just like the new Xterra.
So while we enjoy the large rear windows, that asthetic feature limits its function.