Most of the mountains you’re thinking of have leases from the National Forest Service. These leases allow them to block access to the ski area when it’s operating in the winter because they are doing things like grooming and making snow which are dangerous if you don’t know what you’re doing. They can also stop you from skiing down their trails, even if you hiked in on public land, because if you get injured, you’ll still use ski patrol resources. You’re also using man-made snow and grooming. In practice very few of them actually enforce this, but the possibility is there if people abuse the access.