You forget one important thing - there are rules in place by Marriott to cover early departures:

a) you can be charged the 2nd night stay regardless if you check out early,
b) if you check out a day early it can bump your 1st night to a higher rate,
c) there are no consequences by Marriott to a change in departure date, re: of whether it's in advance or when you're on property - ie, they ok the change w/ no penalty.

As long as those 3 exist, then there are no 'ethics' involved.

Cheers.