Like a lot of folks, I am staggered by the fact that more than 1 in 100 Americans are incarcerated, including approx. 1 in 9 young black men. I don't have the evidence at my fingertips, but my gut instinct, like many libertarians, is to blame the war on drugs for much of the prison population. I would have liked to have seen more detail in the PEW Report on how the population breaks down -- ie for what crimes and sentence lengths -- but no such information is available.
I will say that the PEW report spends way too much time on the utilitarian argument about the costs in public dollars to actually incarcerate these folks. My sense is that Americans almost never complain about the budgetary costs of incarceration. They tend to be more than happy, as a group, to pay whatever it takes to keep felons locked away for long periods of time. I think a much stronger argument is the individual rights complaint that so many people are locked up for what is basically consensual activity.