I think they could've sued that police department. They had no cause, and it resulted in an injury. Nothing says you can't longboard on the road. It's funny that when someone rides a bike fast, no one says anything.
What if they hadn't stopped in time, what if he hit them? That was the dumbest move someone can make. He basically caused an accident. You can even arguee that what he did can be considered reckless driving and endangerment. No words
The cop honestly had no right to stop. They all had saftey gear on they were on a road and not a sidewalk like most states/ towns won't allow you to ride one. What he did put them in danger. That is why everytime you deal with a police officer you get their badge number and name so that if they are not giving just cause for a ticket you can fight against or write a complaint.