Mostly kids on drugs or runaways. At first I thought they were kidnapped or something.
I once saw this documentary about prostitutes. The women were complaining about how they feared for their lives and what the pimp would do if they left the job.
The reporter doing the job paid like a fee to the pimp to set the women free and drove them out of town.
He returned to the same place the next week, and the exact same women were back there. It seems a lot of them are drug addicts, but they tell themselves they are not there of their own choice.