Well it's all very scientific,
in Pokemon, the ice type isn't the frozen water type, think of it more of the sub-zero energy type. (this would also explain the whole water resists ice thing they have going) so the heat from desolate land evaporates the water, but the increase in the surrounding temperature isn't drastic enough to affect the sup-zero "ice type" energy.
Game freak logic
(gām frēk lo-jic) definition: logic and reasoning so advanced that mere gaming mortals cannot
if only I had taught Duskoir ice punch instead...