The truth behind this is not black & white, but more so shades of grey.
The lease allowing for operation on public forest land gives the ski area/resort operator the power to restrict access during times when ski area/resort operations are on-going. Operations can be defined as anything from avy control to grooming operations to other ops work that could be taking place such as an employee snowmobiling from point A to point B. It becomes even greyer when you begin talking about skinning/hiking on trails that have been maintained - trail cutting, snowmaking, grooming, etc. Additionally, the lease agreement gives the ski area/resort operator the power to close access for operational reasons as they see fit.
While many places have no problems allowing respectful and mindful users to travel uphill, they do not have to just because it is National, State or Provincial land. Their lease gives them the right to tell you to get lost if they deem that your presence hinders their operations.