Deceleration Fuel Cut Off (DFCO)- Why don't all cars fully open the throttle valve under DFCO.

When you are decelerating and the ECU cuts off the injectors to save fuel, why not keep the throttle fully open to also alleviate pumping losses? Why pull a vacuum if the fuel is off?I know Mazda opens the throttle all the way, why not all cars? via /r/cars http://ift.tt/2tqr3KK