[Luce]

I don't know about this particular part, but most semiconductors, unless wired up into a complex circuit are "single ended" meaning they conduct in one direction and function by turn on and off. If this sensor is of this type, it can only pull a floating signal to ground, not raise it to supply voltage. It's sort of like a lifter. It opens the valve, but needs the spring (pull up resistor) to close the valve. Or the other much more complex solution would be have a second lobe, lifter, pushrod, and rocker to close the valve. I think some hall sensors have the electronics inside to do all of that, but maybe not all.