He's saying that cameras may reduce complaints by 90%, probably because people think that if there was a camera involved and the cop did something wrong, then he would be punished for it, or alternatively if he didn't do anything wrong, then he wouldn't be. Either way, that person probably thinks it's not necessary to file a complaint.
The issue is that for one, the person may be wrong. Cops rarely get punished. And second, the complaints may drop by 90%, but the abuses may not drop by nearly as much. Let's say maybe they drop by only 20%.
So in practice, this could lead to an overall drop in police abuse, but not by nearly enough to cover all the abuses. However, now it seems the vast majority of people won't even complain about most of the abuses, so those abuses will remain hidden/unpunished, unless you expect the police chiefs or some other oversight body to watch all the videos and then decide for themselves when to punish the cops (which should probably happen - or at least have a "police behavior alert system" that notifies the oversight bodies by scanning the videos).
Cameras were assigned randomly on a week by week basis and complaints also dropped when cameras weren't worn, so public perception is an unlikely explanation.
I think police-worn video will reduce the amount of abusive policing.
But I'm making a big assumption there. I'm assuming that the police know that they're being abusive, and don't want the public to know how they're policing.
What happens if the police are in fact proud of what they do? Convinced that it's the right thing?
For one example see how some police seem to go to maximum escalation at the earliest opportunity. They do this in the name of "officer safety", but ignore the increased danger it poses to members of the public.
