Keith Humphreys points out that “most state prison inmates today are incarcerated for a violent offense”:
[E]ven assuming the best of all policy worlds in which reducing incarceration continues to be a priority, the U.S. is probably too violent of a society to ever shrink its prison population to a Western Europe level. The proportion of the U.S. population that is serving time for violent crimes is larger than the proportion of the Western European population that is serving time for all offenses combined.
Arrests, on the other hand, are overwhelmingly for non-violent offenses. Balko reviews that latest numbers:
According to the FBI, in 2011 there were 3991.1 arrests for every 100,000 people living in America. That means over the course of a single year, one in 25 Americans was arrested.
The FBI also reports that the arrest rate for violent crime was just 172 per 100,000, and for property crimes, it was 531. That means that in 2011, one in 33 Americans were arrested for crimes that didn’t involve violence against another person, or theft of or damage to property. More people were arrested for drug crimes than any other class of crimes — about one in every 207 of us. One in every 258 of us was arrested for drunk driving. The FBI doesn’t keep track, but presumably the remaining arrests were for crimes like prostitution, vandalism, public intoxication, disorderly conduct, and other consensual crimes and relatively minor offenses.