Except that they could never pay it, so it would be a pointless gesture.
I think there’s a case for arguing that parents shouldn’t take children on flights until they’re older, unless it’s absolutely necessary, if they can’t control their behaviour. (I realise that it can’t be avoided sometimes. And it would be impossible to enforce anyway.)
My DD1 (now 17) has additional needs and her behaviour has been very challenging over the years. When she was a young child, we would have found it very hard to get her to sit still in an aeroplane seat, and keep her belt on. It was a battle we avoided by not taking our 2 DDs on a plane until they were 9 and 6.
I would have hated to be in the position of those parents in this situation.