Actually, the energy expenditure of a person with a prosthetic can be twice as much as one without, depending upon the level of the amputation. That means, if the amputation is above the knee, it takes a lot more oxygen to maintain the same gait as someone without an amputation. The other issues people with amputations have is pain at the interface between the prosthetic and the remaining limb. This is the reason many people with that have had amputations choose to ski without it. The pain is increased when walking, but someone may choose to deal with it instead of crutches on snow and ice. This is the reason people with medical knowledge are consulted to determine the rules for acquiring permits and that we shouldn't judge based on quick observations.
I also know that the amputee I ski with doesn't like leaving his prosthetic around because of the a fore mentioned priced so he will often leave it in the car and hop/use his outriggers to the lift. Some places it's more convenient to use stairs then the ramp and or elevator so he will walk from the car. He leaves the pant leg rolled up on the prosthetic though so people rarely give him looks The ski hill is about the only place he utilizes the permit though.