Look up the riots of the late 60's and the LEO interventions.
LE is not willing to put themselves in the limelight the media will cast upon them if they try and hold these idiots at bay.
In the case of the recent riots, and that's really what we have to call them, the police leadership is accused of telling officers to stand down and let them fight. I don't know if that's true or not, but there is at lease some strong circumstantial evidence to say it's a real possibility.