Unfortunately, if Marriott thinks a "correction" is needed, they will do so. And yes, they will do it without notifying you. In fact, their "corrections" usually won't even show up as a transaction to your account. So if you don't keep track of your point balances and transactions, you may very well never realize they made that correction.

So can they do it? I guess the answer is yes as they do it and have been doing it for a long time.