Because vandalism, destruction of public property, and violent lawlessness are generally not well-tolerated in this country.
P.J. O'Rourke had a brilliant line in one of his essays on Somalia- it went something like "In Mogadishu, everything that can be accomplished with guns had pretty much already been done". He was referring to the utter destruction of the city and the society.
We are not Somalis, and we don't settle civil problems by pulling a gun out of the trunk and blasting away. And, having served in countries where they DID "settle" problems that way, I am glad that we don't.