When kids do stupid things, that is harming other people or endangering themselves. And kids do plenty of stupid things. All the time.
Case in point : there’s a 19th centuty moving bridge near my place. There are often kids doing all sort of stupid things when the bridge is being elevated, at the risk of being crushed by the wheels or cables, drowning or whatever else. Should adult bystanders let them do so?
I wouldn’t have much issue with grabbing and chastizing a kid who is, say, damaging stuff, or insulting people, etc… either (let alone running in front of a car and putting himself, the driver and who knows who else in danger like in the linked article).
Last time I restrained a kid was precisely the example given in the article : two boys were fighting on a subway platform. They were maybe 12-14 yo. I separated them, and while another [del] responsible adult [/del] psychopatic child abuser put one of them in the next train, I physically restrained the other who didn’t want to give up and intended to climb in the same train and resume the fight. Should this other stranger and I have instead called the police and, while waiting, placed bets on the winner?