It takes energy to turn the prop, crankshaft, pistons (fighting compression in the cylinders!), valves, fuel pump, generator belt, vacuum pump. Energy is not free, it has to come from somewhere. If the prop is spinning, all this other crap is spinning too. The AOA on this slowly spinning prop is probably significantly increased due to this load, greatly increasing the drag. Remember, AOA on a prop is relative to the blade's chord line and flight path, which is a function of it spinning around in a circle relative to it's airfoil chord line-generally a lot faster mph than the airplane is moving forward, and the airplane's path (lots of people think it's just the airplane's flight path).