From reading the article it says the police had already given a verbal warning to the two juveniles regarding the city's ordinance on skate boarding. Before I go further... Regardless wether or not you agree with that ordinance it is still a misdemeanor offense in that jurisdiction
Ok so a verbal warning was given. What is likely the content of this warning? "Hey kids, you aren't allowed to skate here so maybe head on down to the park or something where it's allowed", to which the kid likely said "we have permission from the security guard", to which the officer will have replied "I don't care, we're telling you it's against city ordinance and you need to stop".
The officer, with prior history and knowledge of the situation, appears to go in for an arrest. The kid pulls away as the cop tries to detain him. The cop tries to gain control of his arm while standing a second time by pulling on the arm. He does not succeed. He then goes to take the juvenile to the ground.
He goes in for an arrest? What could a lone kid have possibly said to the faces of two pretty big police officers that made them think they needed to fucking arrest him? What in that altercation made two fully grown men, police officers nonetheless think "wow, we need to put handcuffs on this 15 year old and put him in our squad car!"?
So even after that, after a fully grown man has decided he needs to apprehend a pretty skinny young kid for skateboarding where he thought he was allowed, the officer thinks "wow he pulled his arm away when I grabbed him - better throw him face first onto a concrete floor".
In summary it appears the cop did what he was trained to do. He went to detain/arrest the juvenile. The juvenile showed passive resistance so he was taken to the ground in order to control him. There were no strikes or uses of less lethal which would have been unjustified based on what the video shows.
He was taught to throw kids to the ground because they skateboarded and then recoiled when he grabbed them?
As I said earlier use of force by the police is not pretty but it is something that is done in order to enforce laws or ordinances which are in place. Remember those laws and ordinances are made by those who citizens elect in order to make them.
You say that as if there would be change to the aggressive and ridiculously hot-headed approach to policing that American police seem to have, should citizens "elect someone else". Sigh.