Possibly because quite a few police departments throughout the country like to model themselves after the military, particularly with the equipment they use.

"This is especially true in cases where, much to the chagrin of civil liberty advocates, police departments have employed their newly acquired military weaponry not only to combat terrorism but also for everyday patrolling. Before 9/11, the usual heavy weaponry available to a small-town police officer consisted of a standard pump-action shot gun, perhaps a high power rifle, and possibly a surplus M-16, which would usually have been kept in the trunk of the supervising officer's vehicle. Now, police officers routinely walk the beat armed with assault rifles and garbed in black full-battle uniforms. When one of us, Arthur Rizer, returned from active duty in Iraq, he saw a police officer at the Minneapolis airport armed with a M4 carbine assault rifle — the very same rifle Arthur carried during his combat tour in Fallujah.

To assist them in deploying this new weaponry, police departments have also sought and received extensive military training and tactical instruction. Originally, only the largest of America's big-city police departments maintained S.W.A.T. teams, and they were called upon only when no other peaceful option was available and a truly military-level response was necessary. Today, virtually every police department in the nation has one or more S.W.A.T. teams, the members of whom are often trained by and with United States special operations commandos. Furthermore, with the safety of their officers in mind, these departments now habitually deploy their S.W.A.T. teams for minor operations such as serving warrants. In short, "special" has quietly become "routine.""

Source: https://www.theatlantic.com/national/archive/2011/11/how-the...