Why bother with cutting the hand if you can cut off the entire arm? What's the use for an arm without a hand? None. An arm without a hand consumes nutrients and is a useless piece of meat, so chopping it off is healthy, no?
Sure, you could argue that such a person could get an artificial hand, and it'd cost less than an artificial arm, but what's the probability that some poor retard will be able to afford it? None. Hell, could a poor retard even work without one hand? No, so why not do a post-birth abortion? Wouldn't it be healthier for his family?
Why do doctors not prioritize the health of society? Why doesn't society operate based on probabilities?