Between service periods, the diner crew may be busy preparing for the next service. This often means cleaning, setting the tables, etc. Once the tables are set, it is impossible to guarantee sanitation of the place settings if those "set" tables are used for casual lounging.
I have to say, that in all my years on single level LD trains (ie, ones that might someday get a Viewliner I/II Dining Car), have I
never seen the dining car set for tomorrow's breakfast, after diner service was over. I mean, tables set will silverware, plates, glasses, napkins, and alike. Never.
If it was, I would agree, but that's not the case. Other than being "reserved" for exclusive usage by crew members, I really don't see why sleeper class people could not use the tables to spend their evening playing a card or board game?