heater and higher RPMs

I noticed that when I select the defroster(windsheild) or the defroster(windsheild)&floor when the heater is on my RPM goes higher. Dont know... but it surely doesnt help with the gas. Any ideas?

Defroster modes include refrigeration for dehumidification. So that means the air conditioner compressor comes on...the engine computer is increasing the rpm to compensate for the additional HP that is needed to run it.
Dave