Just for those of you who have never worked the night shift at a hotel, and I have, sometimes your only duty is to be on site. I worked the night shift at a small hotel where on our busiest night of the entire year, I might have five hours of work to do in an eight hour shift. In the off season, I might be finished with all my auditing duties by midnight (11-7 shift) and have nothing to do except be present in case a guest needed something, until it was time to start breakfast at 6:00. How do you think my post count got so high?
In the four years that I worked the night shift, I don't think there were more than two nights when I actually had eight hours of work to do in an eight hour shift, and on one of those nights we were having the worst blizzard in twenty years.
Having actually worked the job in question, I see nothing whatsoever unreasonable about the poster sitting down. I can assure you that there are no duties he will need to do standing up that he cannot do sitting down. None that would take more than a couple of minutes, that is (perhaps fetching a guest an extra blanket, for example). I see no reason why he should not expect pay for that time; he's still available to do anything that he needs to do.