I imagine there is some "law" enforcement in the barrens. They just don't wear a uniform and a badge

Street gangs would patrol their turf, looking for people causing trouble. (Hard to shake down people if they are getting robbed all the time).
Neighbourhood watch groups looking out for trouble. and what not.
When you are effectively cut off from the services of the city, you tend to provide these things for your selves.
If you want some real world examples, look at Jamaica and their slums, or the barrios of Rio.
In one campaign, the players skipped town and tried to set up shop in CalFree... but choose a immigrant barrens neighbourhood. Within 24 hours of moving in they where met by an very polite East Indian fellow that let them know that "they" would not tolerate violence in the community, but if the Runners where willing to play nice and respect the traditions of the community, they would be welcomed. If they choose not to respect the community... bad things happen all the time. The Players got the message and didn't make waves... and where even able to score a few (low) paying jobs with a few benefits.