I am saying that police can and do abuse innocent people, make false arrests, and sometimes even kill unarmed civilians. And that they are more likely to assume the worse of you and abuse you, falsely arrest you, or kill you if you happen to be young, black, and male.
Would they throw him to the ground if he was polite and respectful? I think they would, but that’s not what is alleged here. What is alleged is that he was polite at first and innocent of any wrongdoing (he showed his valid ID and left when turned away), then they arrested him anyway (why?), then that pissed him off and rightly so, and then he was injured (not clear how, he might have simply fallen).
I have no idea why you refuse to believe that would occur. It does occur. We have multiple instances of police, recorded on video, shooting and killing black men who were already in custody, in cuffs and on the ground. It does happen. As mentioned above, we had a squad of ABC cops pull their guns on a girl here with a case of sparkling water. Actual innocence of any wrongdoing is no protection whatsoever from cops taking unwarranted action against you.
Disrespecting someone who carries a gun - often, but not always, a cop - is not illegal, but it can get you killed.