I certainly agree with you that a contract is a contract and should be enforceable, where there's a mutuality of rights/obligations by both parties.
I think where this situation may deviate from that (whatever the facts are), is that Idaho state law (36-1603) has a provision regarding criminal trespassing and permission forms:
"Permission forms. (1) The department shall produce permission forms for a landowner to indicate that a land user has express written permission to use private land. The permission forms produced must contain spaces for all of the information required by section
18-7008(1)(f), Idaho Code. The permission forms
must state clearly that the permission
may be revoked at any time by the landowner or his agent."
And looks like the IF&G permission form on their website, has that phrase at the bottom: "A property owner may revoke permission at any time."
All of this is hypothetical and not legal advice (nor did I stay at a holiday inn), but:
One scenario could be there was written permission, but the parties did not use the form with the mandatory language about revocation. And, a google search turns up an IF&G "Courtesy Card" that does not have that language. That Card could pre-date any changes to Idaho law cited above, and also pre-date the new permission form. But it's still out there in Google land for people to find and download.