We have been through this ad nauseum in the past and they would be completely within their rights to restrict hiking on the ski trails. Not the mountain itself but they certainly can restrict access to facilities and trails under their management.
Right. Ski area operators are given the right to "close" (restrict access to) anything within the area boundary/within their special use permit. Many many many areas do not allow hiking and skiing during pre-season snowmaking. And they have absolutely no obligation to post signage to that effect. If management asks you or tells you not to ski it, you should probably heed their word.
Lots of people were kicked off of A-Basin out here while they were making early season snow. It just makes sense, from the resort operator's perspective.