They're not volunteering to do the work of a ski patroller. Thats fucking bullshit. 99% of the time, its simply a police officer who wants to go skiing. They get to ski for free if they ski in their uniform, as long as they uphold the law when they are there.
Whitewater, one of the most laid back and carefree ski hills in Canada has had RCMP officers go skiing there. All they have to do is carry a radio and response when they get called, and more often than not they get called for theft of services (people using other peoples season passes, or trying to ski without a lift ticket). Some Cops are skiers, and some shred pretty well.