This a byproduct of the 80s war on drugs, 9/11, and the creation of the Dept. of Homeland Security. With billions of dollars now available to local police forces, and "surplus" military weapons including tanks, drones, and full-combat body armor, some local police forces look more like assault troops than their "Serve and Protect" motto seems to indicate.
Infra from http://www.vindy.com/news/2012/jan/22/police-are-transforming/?print
To our detriment, local police — clad in jackboots, helmets and shields and wielding batons, pepper-spray, stun guns, and assault rifles — have increasingly come to resemble occupying forces in our communities. “Today,” notes Paul Craig Roberts, former Assistant Secretary of the Treasury and associate editor of The Wall Street Journal, “17,000 local police forces are equipped with such military equipment as Blackhawk helicopters, machine guns, grenade launchers, battering rams, explosives, chemical sprays, body armor, night vision, rappelling gear and armored vehicles. Some have tanks. ”
Yet appearances to the contrary, the American police force is not supposed to be a branch of the military, nor is it a private security force for the reigning political faction. It is an aggregation of the countless local civilian units that exist soley [sic] to serve and protect the citizens of each and every American community.
It is particularly telling that whereas in the past, law enforcement strove to provide a sense of security, trust, and comfort, the impression conveyed today is one of power, dominance and inflexible authority. However, this transformation of local police into military units did not happen overnight. It cannot be traced back to a single individual or event. Rather, the evolution has been so subtle that most American citizens were hardly even aware of it taking place.
What began with the militarization of the police in the 1980s during the government’s war on drugs has snowballed into a full-fledged integration of military weaponry, technology and tactics into police protocol. As Becker and Schulz report, more than $34 billion in federal government grants made available to local police agencies in the wake of 9/11 “have fueled a rapid, broad transformation of police operations ... across the country. ... Police departments around the U.S. have transformed into small army-like forces."