I think everyone knows they shouldn't work without a work permit. The ramifications of being caught occur to some people, and are less compelling for others. The apparent likelihood or otherwise of being caught is one factor in people's decisions to do it.
You are probably unaware of this, but for young people from NZ and Australia (for whom a year or two of international travel and "informal" jobs has been almost a traditional rite of passage for many decades) it is (or at least was, and I can't see any reason why that might have changed) very difficult to obtain US work permits, particularly once they are in the country. The way many of us have traditionally travelled in our youth has involved a bit of ad hoc decision-making which is heavily dependent on funding, friends, timing, and possibly snow conditions in relevant cases.
Here's a hypothetical scenario for you. So, say you are hitching (OK, I wouldn't have but it probably happens) from California to Colorado, with the intention of sleeping on a mate's floor in Steamboat Springs for a day or two, then moving on to, say, Florida. Instead, you fall in love with the place, and decide then and there that you will stay and do a bit of skiing. After a day or two of paying for lift tickets and rental gear, it quickly becomes apparent that you will run out of money unless you get a job. It just so happens that Mr and Mrs Winterover's nanny has broken her leg during a nasty fall while accompanying the kids down a mogul field, and they need someone to replace her until her leg is healed. You have the necessary qualifications and experience (first aid certificate, half-way through a teachers' training course, or whatever) and for whatever reason, they're not overly concerned about the legal aspects. So you're sorted, and their dilemma is solved.
But anyway, this is a discussion about how we women take our ski holidays. I made a throwaway comment about something I did a long time ago which I didn't really intend to elaborate on. I will say though, that I got that job at the end of a week's holiday in a self-catering apartment with a friend and a bunch of other people we didn't know. It was partway through the season, and the girl I replaced had just broken her arm and gone home, unable to ski. The employer was short-staffed and desperate.