Speculative, without circuit design details, but experience from other unrelated electronics;- they'd have to decide on a resting state to leave the earth switching transistors that trigger the coils to fire, which could be either way, connected or not when the control power input is quenched.
Logically, if they removed the earth path on power down it would discharge the coils, only to pick it up from zero when it's switched back on.
Appears like a safer option to start the control,electronics without any residual power anywhere to interfere with initial logic or component lifing.
So turning them of would charge both coils (on these four cylinder) without the crank spinning, only to then discharge on shutdown with nothing being used in between.
