"Just talking to them" doesn't always work (e.g. toddlers, children with pathological demand avoidance, etc), but I agree that abuse is still never the answer.

Adults don't always understand. Sometimes force is actually required.

Something tells me the parents tried that already with this kid.

Kids are smart. But they don't always have the same thought process as adults. There have to be consequences for actions. And a stern talking to just isn't going to cut it for a lot of kids.

Punishments like spanking give an immediate reinforcement and I wouldn't consider that abuse.

Different situations require different ways to handle kids. On an airplane where the kid won't sit still, I would certainly consider something immediate that doesn't need a lengthy explanation.

Usually yes, and I will acknowledge that skipping this step is showing laziness or incompetence, but I don't think children are already reasonable, especially when they're tired (this happened late in the evening) and already committed to throwing a tantrum. And in this case the whole situation might have been already stressful for the parents anyway, which doesn't help. Probably they should have talked about it more way before they entered the plane, and prepared the kid for what was going to happen. And maybe they did. But the situation then being what it is I think options can be very limited, and there probably wasn't much time to try reasoning with the kid