Yeah I don't know how far back your experience goes but this has somewhat recently changed. It used to be you could say "I want to pay for this drink" as you were getting close to your 15 Cheers drinks, and you'd pay out of pocket for the drink and it wouldn't count against your 15 included in Cheers. And people buying drinks a la carte could buy as many as they wanted.
Now, you are only served 15 single serve drinks per day. It doesn't matter if you order them via room service, from a bartender, or a roaming waiter. And it doesn't matter if you have Cheers or not, you only get served 15 drinks a day.
Limited exceptions - like buying a pitcher of margaritas or drinks purchased on private islands - apply.