I would speculate that the compressor they used doesn't have the umph to make a decent heat pump, so a resistance heater element would be necessary anyway - and someone figured the additional cost of engineering the heat pump system was too much to bother with. Designing heat exchangers that work as good evaporators AND condensers is no easy task, and the locations of the components would have to change. A system designed to both heat and cool would necessarily not be as good at either task as a dedicated system. For example, you couldn't include a subcooler with a heat pump.
They were probably banking on heated seats and steering wheel for comfort and the heater was designed more as for defog/defrost than cabin heating.
The inclusion of a water circuit seemed really weird at first, but it makes a little more sense after reading the description in the service manual. The heater is controlled by PWM, so the liquid acts as a nice buffer to keep the temperature steady at the desired setpoint - much easier to control.
I maintain, though, that there could have been a mechanism to heat the cabin air with waste heat from the motor/inverter/charger cooling circuit... I don't know how hot that fluid gets, but if they're actively cooled there just might be something! Anyone willing to measure the main circuit coolant temp this coming winter?