I don't think it is anybody's intention to stiff the wait staff. It is just nice to come back from a day ashore, or come back from dinner, and sit out on the balcony and enjoy a nice libation as the sun sets over the horizon. Sometimes it not about saving money, but simply a matter of convenience - better than looking for a bar, buying a couple of drinks and having to carry two glasses up/down the evevator, through the hall and banging on the door (because your hands are full)and arriving without having spilt half the drinks on the hallway carpet.
I don't know if you could order room service, but then you would have to wait (and stay dressed) for service to arrive.
There needs to be a reasonable balance between the ship needing to make a profit and cutomer satisfaction.