[avi8torny]

Couple reasons....

Deice is a huge electrical consumer on the airplane. So the crew must have the ability to shed it in the event of a fault or because of some loss of the supply. Another reason to turn it off is on the ground, the deice (pitot, AOA indicators, static ports, ADC sensors fuel vents, engine intake lip ice, heated props, windshields etc) will get really hot....and with no airflow it will cause damage to it or something else.

I'm sure there a couple of more reasons.

Hope that helps