Probably by having the panopticon-effect.

The same way a surveillance camera in every room would also reduce bad behavior...

Only way to achieve that is if people are afraid of these forces and perceive them as effective at policing.

If they're not, nothing will happen.

And considering that the US has the highest murder rate among first world countries, highest incarceration rate and spends the most on the military. Obvious the US is doing something wrong.