Pretty sure that if your hands touch the snow, you lose points. Same thing's the case in slope and big air comps. Guys get deductions for touching their hands on a landing. Aerialists are taught to land with hands forward and out as much for stability as to clearly show that their hands didn't touch down. Also, as said above, demonstrates control.
They always throw their hands up to hype up the trick. I guess I never thought of it that way, but yeah, it is like the aerialists version of Afterbang. It would be funny as shit to see one of them totally afterbang a landing and thug out some heavy head on the way down the landing hill. Problem is that the skis they use are pretty short and not center mounted so if they leaned back on the landing, they'd fall on their butts.