This may not be a popular opinion, but - typically, if you are hosting, you either serve alcohol or you don't. Cash bars are a no-no. Now, a memorial is a different thing from a wedding or cocktail party, but you may want to consider one of the two following options. Either a) serve only soft, with no bar in the room - anyone who wants to wander over to the hotel bar may do so - or b) pony up for softs, beer and wine. I wouldn't have a full bar in the room if guests are expected to buy drinks - I wouldn't expect to pay for anything at a memorial service (valet service is an exception - that's out of your control).
It could be completely out of the hosts' hands, though. While I realize that a funeral reception and a wedding reception are not at all the same thing - the facility where DS and his fiancee are thinking of having their wedding stipulates that in the absence of a hosted bar, the facility reserves the right to set up their own cash bar in the room. (This facility does have several very large rooms, probably none small enough for a funeral reception, but it might still be a common practice among hospitality-providers for any kind of event).
So, even if the hosts want to provide just punch and soft drinks (perfectly acceptable), the restaurant might put a cash bar in the room anyway - and of course people will always remember that Mary and Mark "had a cash bar at their wedding".