I'm coming down on the old man's side here, for a couple of reasons.
1) Safety for others? How is skinning up more dangerous than a child moving at 3 mph in a wedge? It is the responsibility of the downhill skier to avoid obstacles.
2) Safety for himself? Live free or die.
3) It's not just public land, its US Forest Service land. Easterners may not get that emotions run deep around this issue, but most land out here is USFS or BLM, and it is OURS. YOURS. Much of it is leased for various purposes. These leases are very specific -- eg my uncles lease tens of thousands of acres of BLM land for ranching, but they cannot tell people not to hike or hunt it. They lease it for grazing, that's it. Maybe JH lease specifies that you have the right to kick people off the slope, maybe not. It would seem strange if it was a State (WY) law since its federal land.
I used to cautiously skin up ski resorts to get to the backcountry, especially at Mt.Hood, and never had a problem with it until at a little resort in OR (Hoodoo). I was on a little used side slope, when a patroller asked if I had a ticket -- I said no it was forest service land, and I wasn't riding the lifts. She said I still needed one. I ignored her and she went away eventually -- I went uphill, gravity took her downhill. I didn't ski over her skis. No major fuss. Later I found that they don't check tickets on the backside so it made sense, but only for $$ reasons. I don't know about their lease.
Once when I lost a ticket on the backside of another resort I was forced to hike out -- so apparently there was no safety issue in going uphill in that case.