I would love to know what happened to those 3 after the camera stopped rolling. It sounded like they where in America, so, if I'm not mistaking, what that cop did was illegal.
Not exactly... it is dependent on the situation. Yes in this country we have freedom of speech, etc. but we are still not allowed to disturb the peace. What the guy flipping off the cop did was illegal and in some states an arrestable offense therefore the police officer can use as much force necessary to effect the arrest and end the aggression.
In this case, due to the guy's position when flipping off the cop, I would have said that I was afraid he was going for my sidearm which allows for a greater amount of force to be used. When the guy stood up and ran back to the car it becomes felony fleeing (again depending on the state) which allows for additional force to be used.
Ultimately the police officer did not follow the spirit of the law but can probably be ruled justified on the letter of the law.