I believe the reason for this is emissions related. The idle is kept high while leaning out the fuel mixture to lower emission. When the throttle blade slam shut, the fuel mxture is still very rich. This is checked during a smog test. Here in Massachusetts, the cars are accelerated on a dyno at a certain rate to 30 MPH. At 30 MPH, the gas is released, thus closing the throttle blades and causing the emissions to go up. If you don't let the car idle down, you won't get that spike in emissions. This is the way car manufacturers get around smog tests. Car pollute worst at idle.