Unless the lounge is full with all tables occupied, a couple sitting at a table that would otherwise had been unoccupied and ordering a couple of inexpensive beverages doesn’t reduce the waiter’s income as otherwise he would have had one less customer. Nor does ordering a couple of beverages take much of his time.
And, additionally, even if it did that is simply the nature of the business and they know and expect that. Some customers come in and order expensive wine and tip accordingly, providing an income far in excess to the effort in serving the customer. And some customers order little and thus don’t pay much. If the joint didn’t accept that they would have instituted a minimum charge for sitting.