Unsupported statements about Marriott's policies are not persuasive, particularly when they fly in the face of actual evidence that many Marriott properties regularly encourage government employees who are not traveling on official business to book the GOV rate. Marriott has every right to deny the GOV rate to unofficial travel across the board but instead has decided to leave it up to individual properties. If this were not true, Marriott's general policy statements would be different, and the various "rate rules" described in this thread would not exist.
By simply saying "no" without offering any evidence, posters in this thread are not disagreeing with this argument; they're just being disagreeable.
What Hilton, Starwood, etc. choose to do is not relevant to this discussion but, FWIW, my co-workers regularly receive offers from various hotel chains offering them special rates on leisure travel as a courtesy to anyone attached to any form of public service.