I agree with qtiger.

It's to promote smooth steering wheel movements. I find that lot of the beginners literally hack at the steering wheel.

I told this one person to go slower and make precise and smooth steering inputs and he dropped his time by 2 seconds.

having said that I've crossed my arms many a times in an autox. some times they like to put these hair pin turns where you have no choice. Either that or start the turn with one hand at the 12 O'clock position.