The emergency stop is for the user or rescuer. The disconnect is for service personel. Different things.
The disconnect removes all power, the emergency stop only needs to stop the motors. It is possible to make this one switch but that is not usually how commercial systems work. If nothing else, the emergency stop usually leaves the light on. I have seen some old systems that only turned off the jet pump (the low speed circulation pump and heat kept running) but that is not really legal.
