Sure enough, the GM’s response on TA for OP’s stay precisely mirrors the feedback already given here:
Not sure why this surprises You, this does appear in every confirmation email of every reservation booked with Marriott. What our staff mentioned to You at check-in is that there are no "penalties" for early check-out (which some hotel chains do charge), but that doesn't mean that the terms and conditions that appear in every confirmation email of every reservation booked with Marriott would no longer be applicable.
Unfortunately in the Las Vegas market many people make multiple-day reservations (because the price goes down for multiple nights) then check-out early expecting to get the lower rate. Not saying that this was Your intention, but that is why the terms and conditions made by Marriott are what they are.