I don’t trust that any modern company would actually “use the surplus to subsidize products for poorer consumers”, that sounds like the kind of College Economics fact that isn’t actually true in the real world.
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.
It’s not so much “subsidizing” as maximizing revenue by charging as much as each individual consumer is willing to pay instead of a flat price. Albeit the outcome is very similar.
It's the sort of thing that a strong regulator can mandate, but generally doesn't; and the sort of thing that an airline would promote heavily, but not in a way that anyone could rely on: "We donate tickets to Charity X who uses them to fly very sick children to world-class hospitals! Why not donate your airmiles right now, relieving us of some of this obligation without getting the usual tax deduction!"