It's because, as you are descending, the chance of forgetting to turn on the ice when the temperature warms beyond the -40ºC limit is to great to worry about the tiny amount of extra fuel it takes to turn it on as you enter the cloud.
Puuuuuure bollocks
Have you heard of ice crystals at temps way below -40c?
During descent at low thrust,these crystals will accumulate in the compressor area and bang bang here goes your stall/engine fail.