There is another legal problem that this technical solution doesn't address. User A racks up 100 miles owed. Then says FU, and deletes their account. They create another account and continue to rack up miles. Who will take action against A? The exchange will send them a stern email?
Or, rapists/criminals determine that this is a good way to get their targets to lower their guard, and cities are faced with another crime vector. Who pays to enforce?