Some of the falconry lads might be able to answer that best, but I'm it's definitely not just younger birds learning, and I think Sparrowhawks feet are designed more for catching and holding rather than puncturing and killing, therefore as soon as bigger prey is immobilised they start to eat it. Also why waste energy killing when it can eat it alive? And time spent on the ground with prey they can't lift, is time when they're most vulnerable to other predators?
I've always just wondered why they do that and probably another reason i'm not to keen on them. Have they got any predators really?