I have only tried "free" drink a couple of times but it seems that some crew members are not aware - once purser had to instruct the cabin crew member serving me, another time they said "only one drink" etc. On their payment machine there is an option in menu which is called "Diamond consumption" where they are supposed to bill those "free" drinks.

Particularly on my last HAM CPH (CRJ900) they have only served Plus class and no service at all in M, announced as "due to short flight duration" - it might be somehow connected to tread starter issue.