Let me propose another analogy--One that I have seen many times when watching the security-instruction videos which play in Amtrak stations. These promos let you know that your bag may be subject to random checks. If you choose not to have your bag inspected--for whatever innocuous or nefarious reason you might have--you won't be allowed to board the train, but you can go back to the ticket window and get a refund. And if memory serves, you're within your rights to decline to have your bag inspected by the similarly-random checks on the subway . . . but you won't be getting into that station.