You can't help it.
It's a reflex. You stick out your hand when you fall - it's better to stop the descent with your palm than your face, right? That's fine when you're ambling down the street, but add an extra 80 mph and the weight of a motorcycle to the momentum equation and the bones in your paw might not take the strain very well.
Source:
http://www.wired.com/reviews/product/pr_fetish_knox_handroid?ts
