My guess is that they see their device controlling a heating element of unknown (to them) provenance as a liability. My zen15 manual specifically has a warning against using it with heaters.
CAUTION This is an electrical device - please use caution when installing and operating the Power Switch. Remote control of appliances may result in unintentional or automated activation of power. Do NOT use this Z-Wave device to control electric heaters or other appliances which produce the risk of fire, burns, or electrical shock when unattended.