So the clunky relatively expensive AC switch on the front panel isn't actually connected to the supply? That was the actual question, quite amazed I had to spell it out.
Someone RE'd the mains-side schematic for the 852D and posted it in the Sparkfun comments linked above:
The mains switch is a DPST, one pole for the heater and the other for the pump and controller. There's a triac across the pump/controller pole so the controller can keep the pump and itself powered when the unit is switched off so it can run the pump to cool down. Then it can turn itself off and stay off. Even if the triac shorts out only the pump and controller will be powered, the heater is positively disconnected with the switch off. Rather clever design.
I haven't seen any schematic for the 858D/898D but it does appear that they have "soft" power switches and rely only on the triacs to control the heater.