The welfare of sled dogs has increased dramatically over the years, and I don't think anyone would suggest that there isn't still some room for growth.  But why single out mushing?  The average sled dog is almost certainly safer and healthier than the average pet-bred dog.  You could get these same shots multiplied many times over of pet dogs...piles of dead dogs after an overcrowded shelter euthanizes all the unadopted animals, neglected dogs chained in yards, terrible conditions at puppy mills, dogs allowed to become painfully affected by lack of grooming, abusive trainers and owners, etc.  Let's be honest - it's not mushing that's the problem, it's our treatment of domesticated animals in general.  At least there is some accountability for competitive mushers - there are not the same checks on regular dog owners.