They have been militarized and become a dangerous lethal force trained and conditioned to serve and protect only the political establishment by maintaining order -- discipling the population -- and therefore permitted by that establishment to function largely without accountability.
Nothing is done to curb law enforcement excesses because of the nation's partisan political divide. When conservatives control the political establishment, they unleash the forces against their political opponents. Liberals decry that but adopt the same tactics (while conservatives complain) when the liberals are in power.
Those inclined to disbelieve or doubt the above, or to think it exaggerates the situation would do well to take a look at and consider this roundup of some of the nation's recent tragedies. And for a detailed description of one scary but not unusual episode, take a look at this report.