because its almost impossible not to
Exactly. Seriously go try it. Get one of your friends to skate down the ice and you hit him head on. Try not to:
a) brace for the impact by pushing up and into the hit
b) not use your arms to 'throw' your friend away from you after impact
It's called center of gravity. You have to brace that way in order to stop yourself from being thrown backwards and injuring yourself. For every action there is an equal and opposite reaction, that energy must go somewhere. The best hitters direct that energy back into the person they are hitting, causing the biggest hit. If Stuart were to absorb that energy himself he takes the risk of injuring himself or falling down and being unable to rejoin the play.
People who have a problem with that have never hit someone like that in a game, plain and simple. If you had you wouldn't even question what Stuart did, it was textbook.