The heavy-handed tactics were probably used out of fear that the protests would turn into destructive rioting, as happened in Los Angeles after the acquittal of the police who beat Rodney King.

The LA case was fueled by broadcasting selected parts of a video which showed the police beating King but omitted the parts where he kept getting up again and attacking them. The jury saw those parts, but the mob didn't.

In Ferguson there's no video, but there are witnesses claiming that the policeman shot Brown repeatedly for no reason. If that's true then of course he must be prosecuted. But it sounds far-fetched. Why would he do that in broad daylight in front of witnesses? He would have known the consequences, and he would have been aware that someone might very well have been video recording the incident. The policeman's account of self-defense is more plausible, especially given Brown's size and the store video showing him grabbing the clerk by the throat and throwing him against a display case.

Racism is central to the Ferguson case, but it's not necessarily the racism of one town or one policeman. It's the racism of a society that kept Africans first as slaves and later as an underclass based on race. Incidents such as this are the inevitable result.