I think it's because most people broke the knobs off them...it's difficult to turn the knob when the throttle is in the normal idle position.
It's more accurately a pre-start-of-engine idle adjustment knob.
With the car off, you step on the accelerator pedal to move the throttle linkage, taking strain off the bowden cable...then you turn the idle adjustment knob to shorten the bowden cable...thus enriching the idle when you finally start the car.
You couldn't speed up the idle with the knob while the car was already running...unless you simultaneously feathered the accelerator pedal just the right amount.
It wasn't a "fool"-proof system.
And..who reads the instruction manual anyway?
