Your knowledge of laws regarding government rates seems fuzzy. Marriott offers government rates as a courtesy to government employees or as a marketing technique. If they choose to ask for an id at check-in and the guest cannot produce one, the worst thing that happens is that the guest pays a higher rate. The GSA does not send out the Inspector General and noone is accused of impersonating a federal officer. It's like signing up for the AAA rate and not having an AAA card when you show up. I don't recommend it- you may end up paying far more, but it is not breaking the law.