[addingnumbers]

> They can easily use safety under “weights and balances” as the reason.

How would it play out for them to invoke that law disingenuously?

They won't use it to fly with an empty seat, nobody wants that except maybe a couple adjacent passengers who might enjoy the elbow room. The airline certainly doesn't want an empty seat.

If they stated out loud to someone that weight in that seat puts the passengers' lives at risk, then seats another person there, I'd hope the backlash would be instant and severe.