That’s not the way to look at it. The airline has a total cost hurdle for the flight to make sense. If they don’t cover that the flight doesn’t happen. So in a very real sense, your seatmate paying half what you are could be the difference between the flight going or not. As long as they are covering the direct costs of their flight (fuel, etc.) they are helping the flight take place at all. Since the assumption is the business travelers ‘really, really want that flight’ everyone benefits here.