I just want to add that just because a ski resort is on "public" land most definitely does not mean that anything goes.
If it's on National Forest land (most common here in the West) the ski resort is operated under a special use permit. For almost all legal purposes, this permit gives the operator the right to limit any and all activities pursuant to the terms of the permit. It essentially becomes the OPERATOR'S property, not public property.
The operator pays a fee and for that fee receives the right to make rules that apply to anyone using the property. If the operator wants to say that hiking up is not allowed and that policy is not prohibited in the contract, then that's the rule the public has to follow. The same concept applies to selling hot dogs and beer or giving ski lessons on the property. So, if the ski area operator wants to make a policy stating that you can't hike up the hill while the area is open (or any other time, for that matter), they may be completely within their rights to do so.
Sorry for the rant, but this is kind of a sore spot with me. There seems to be this assumption that because the LAND is public, therefore the public can do anything they want. That's not the case.