The issue is, if they only adjusted the rate down to the going rack rate on day of arrival, then the OP would likely be paying significantly more for that standard king room then they would've been able to book it at with a discounted rate or in advance. Its not entirely clear if this is a same day reservation or not, and if so then it is clearly bad inventory on Marriott IT or the hotel.

Now with some airlines, if you pay for a first class seat and they downgrade you, you get the fare difference at time of booking, as well as a goodwill compensation for being stuck in something less than you expected. The hotel should've offered something in addition and the manager been ready to inform the guest at the front desk and provide the offering. Letting them check in without the slightest information on the downgrade and only admitting to it when they get caught after the OP enters the room amounts to bait and switch.

Oh and please provide the name of the offending property. Others need to know what location it is when properties pull stuff like this.