Doing something illegal, not charged, but here's the thing: why the handcuffs?

Doing something illegal, not charged, but here's the thing: why the handcuffs? Is this procedural? If so, why? Do we presume everyone responds to interactions with the police by becoming violent and therefore the police must pre-emptively handcuff everyone to protect themselves? Is there no discretion with this? How would you feel if someone handcuffed you until someone else came and got you over, say, a parking ticket? I really wonder sometimes about what we've decided our society should be.

https://www.usnews.com/news/national-news/articles/2017-06-23/teens-handcuffed-for-selling-water-on-national-mall
https://www.usnews.com/news/national-news/articles/2017-06-23/teens-handcuffed-for-selling-water-on-national-mall