No matter how many excuses you people make, the simple fact remains. If she would have done what she was told, none of this would have happened. That doesn't excuse what the cop did, assuming he was even in the wrong. It was her fault for misbehaving. Period.
And if the cop had shown a reasonable reaction to her teenage defiance, he'd still have a job.
Alas, he wildly overreacted. And 'Officer Slam' won't be working around kids in the future. Thankfully.
The phrase to keep in mind is 'necessary force'. It wasn't necessary to grab her by her throat and slam her on the ground in order to resolve the situation.