Having restrictive agreements to prevent track owners from renting out their circuits for non-sanctioned events would, I suspect, be ruled a restraint-of-trade if it were ever legally challenged.
Motorsport UK, which deals with 4-wheeled racing, has long since given up trying to prevent unsanctioned events from using MSUK-licensed circuits, or MSUK officials and competitors from taking part in unsanctioned events. As both a competitor and official I'd choose not to compete in unsanctioned events because there are legal ambiguities around the RTA and potential insurance liabilities, but neither do I think sanctioning bodies should have an effective monopoly or be allowed to shut out other sanctioning bodies from circuits.