Yes it is fair to charge higher for single players at night.
What if there is a waiting list, why should room owner lose money when a single player ties up table at a single player rate when they can rent out at 2 or more player rate which would be much higher. So the compromise is, charge the single player a little higher where they probably won't play as long. That is only fair.
I suppose that's "fair" from a room-owner perspective, but it's a good way to drive away people that play by themselves. It's not fair to customers, in my opinion.
I see your point, and like I said, if I were a room owner and there was a waiting list for tables, the guy playing by himself is "taking up a table", but then again, he's probably going to be there a while; which is still generating revenue.