While I'm not a fan of skateboarders myself, screaming at a 14 year old who is standing around doing nothing dangerous is a lil excessive. "When I'm talking, you shut your mouth and you listen." "The sooner you learn that, the longer you live in this world. Because you go around doing this stuff, someone's going to kill you." Personally, at that point, I'd be worried that he was going to flip out and do something nuts.
Assault a kid, stealing his board, hitting him again. Then screaming at him. I suppose a lot of folks would consider this properly disciplining a kid. If you're the kid's parent, sure. But if I was that kid, I'd probably have a healthy fear and loathing of police officers afterwards.
At least he learned an important lesson. Never trust a government. Ever.