Question: how can the airline afford this ... ?
Answer: because it's not going to cost them anything.
It's not that they won't sell the seat if someone wants it; it's just that if there are any "unsold" seats on the aircraft, they will sell one to you (for $20). So, if there are unsold seats on the aircraft, and you don't pay them, they will make sure the open seat is more likely not next to you.
It's kind of like the way the Mafia used to sell people insurance (you paid the Mafia to insure that the Mafia wouldn't kill you).
Airlines are getting briliantly creative at selling you things that don't cost them any money.
A few years ago, Robin, my partner, and I flew from Oakland to Denver on a Frontier flight. When we purchased the tickets, they would have charged us something, I don't remember how much, to select seats. So we opted out; it was only a 2½ hr flight. and we felt we could be separated for that long. My seat assignment was about 29 rows behind her.
When we boarded the aircraft, and the flight crew discovered that Robin uses supplemental oxygen, and that they might have to assist her if she was sitting by herself, they miraculously discouvered that the middle seat next to her was empty, and moved be into it, so I could assist her with the oxygen.