If its like any of the other Boeing logic - the only time heat would be removed from the window is if its in the ON position and its warming up too much - ie sitting on the ramp in a warm climate getting direxct sunlight on it.
It then removes power to the window but the window heat is technically still on
Once airborne both altitude and airflow cool the window and it would return to heating the window again.
It has less to do with the voltage applied as it has to do with the heat of the window.
I suspect this is not a Operational procedures fail but perhaps a batch of poorly created windscreens.
Even if the Window heat was left off on the ground and only noticed at altitude - then turned on it would not immeadiatley shatter the windows - the heat slowly warms the window in a gradual process and is no different than turning it on, on the ground during a cold winter day in the northern latitudes.
WJP