For some reason, I happen to think of Kent State (You do remember Kent State, don't you?) rather than the riots in Washington, D.C., even though I live just across the river (plus about 20 miles) in Fairfax County, Virginia. In that case, the National Guard was on hand but one point I want to make is that after two or three days of trouble in town, before the National Guard showed up, it is said "there was shotgun behind every window in town." That is undoubtedly an exaggeration but it shows that the situation had gotton out of hand in the eyes of the townspeople.
Another point is that, whether you think such people are cowardly or badly led or mislead is neither here nor there. In that particular case I believe the individuals involved as rioters or demonstrators failed to realize the seriousness of what they were doing or threatening to do--but maybe they did. Their motivation was very likely to have been a lot different than the rioters in France at the moment or in D.C. in 1968 (or where ever you care to mention). Another obvious factor is the supposed anonomity of the rioters, as individuals, a factor reinforced by the way that many rioters wear masks or otherwise cover their faces, so that they may attempt things they would not as individuals, for whatever reason.
Shoot low, sheriff. They're riding Shetlands!
Underneath the starry flag, civilize 'em with a Krag,
and return us to our own beloved homes!
Buy War Bonds.