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1. The engine is still compressing air. If you were to kill your engine while driving and open the throttle you would find you get an increase in engine braking cause you are allowing the cylinders to fill completely.
2. You just described "pumping loss". One major inefficiency on the gas combustion engine.
3. Lack of direct connection is the main reason. Autos are also geared taller which results in lower engine speed during engine braking. The torque converter multiplies torque output to compensate for the taller ratio during power demand.
4.Just drove a Jetta TDI and I almost ended up hitting the windshield when I let off the accelerator. Trucks hauling heavy loads employ an exhaust brake. This is a throttle like valve in the exhaust collector that increases backpressure in the cylinders to amplify engine braking. Diesels burn far less fuel than gas motors at idle and partial throttle because they do not employ a throttle valve and therefore less pumping loss.
5.I use fuel cut decel alot. I am not a hypermiler but I do try drive conservatively.
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