The United States has areas of significantly bad crime. 4 of the 50 most dangerous cities in the world (measured by murders per capita) are in the U.S. These cities have murder rates from 44 to 60 per 100,000 people.[0] For reference the United States as a whole has a murder rate of 4.88, and United Kingdom has a rate of 0.92.[1] Making matters worse the violence isn't distributed evenly in those cities, it's often concentrated in a few "bad parts."
There are neighborhoods in the U.S. where your chance of being the victim of a violent crime in given year is 1 in 7.[2] That's for civilians. When we send police in to try and protect those victims it's hard to send them unarmed.
[0] - http://www.worldatlas.com/articles/most-dangerous-cities-in-... [1] - https://en.wikipedia.org/wiki/List_of_countries_by_intention... [2] - http://abcnews.go.com/Business/worst-neighborhoods-violent-c...