This behavior of the AC engaging in Auto mode is not unique to the Murano. In fact, I believe that any car with climate control will engage the AC condenser, as it does indeed dry the air. My wife's 1996 Bonniville does the same things, as does my mother-in-law's '02 BMW. In many cars, it will auto engage when you turn on your windshield defroster, and you cannot turn it off. I remember my first car did not have AC, and it was literally impossible to keep the windshield from fogging up during a rain or snow.
Made it very difficult to cover your tracks after excessive backseat activity :22:
But seriously, this is common among cars with climate control.