The officer had his knee on a man's neck. The man was handcuffed. What harm could the man had done, being handcuffed?
Virtually none. He could have been sat up, against the police car with no risk to anyone. And this would have almost certainly have been the case if he was white. And let's not forget that his "crime" was apparently using a dodgy $20 note. He had no weapons, was not resisting, and was no threat to anyone.
And for those (not you Dave) saying why here, why now?
Imagine that during the 80's, when football hooliganism was far more prevalent, if football fans were being killed by aggressive policing. Imagine that this was excused by others who pointed out that football fans were scum, that they would often fight with each other anyway, and therefore they somehow deserved everything they got. Imagine then that at a club far from here, one fan, a family man, was killed by overzealous police on his way to a game, and that was filmed and shown over and over again on all media forms.
Would we as football fans stand up to this, and protest that we were treated as nothing more than second class citizens, simply because of our choice of sport?
If that movement was called Football Fans Matter, would you not get involved because it wasn't our club? Would you moan and say what about Rugby Fans? Don't all sport fans matter?
It really shouldn't be hard to understand.