It has a lot to do with prison over-crowding, and police brutality. People have a tendency to go to extremes, particularly if they've been a victim of "stop and frisk" or police profiling. The difficulty is - there's a tendency to want to throw the baby out with the bathwater so to speak. It's not really libertarianism per se, so much as a fear of the police state, and a deep-seated conviction that most people imprisoned are innocent or in for minor crimes, or racial profiling. This is true to an extent - but not completely. Yes, POC are imprisoned more than "Whites", and yes, they are put to work without pay or compensation. But, that's something for systematic reform - and not just prison reform, but Justice system, courts, etc. And that's hard to do with the current conservative judicial appointments - so in frustration, the left has opted to get rid of prisons and police, because they can't change the justice system itself, which is admittedly racist and imbalanced in the United States.