I didn't see the show but because i work in human services can be pretty safe to guess that young girls prostitute themselves (this is not sex slave situation) because it's all they know. They think themselves worthless, useless, have been sexually abused by men as children (there's the false sense of being loved- dad, brother, whoever, is only nice when...otherwise he's hitting yelling, shaming and degrading...). They verbalize this by saying they need the money (which they do if runaways). Or by saying it doesn't bother them emotionally.. etc. They are able to rationalize their behavior in the safest way possible which is to disconnect from the sexual act/have no feeling about it. Unfortunatly, there are human feelings about it and as successful as they are with stifleing the feelings, it is a potential volcano of hate and anger at themselves, strangers, and family members who failed to rescue them.
This is not my field of expertise! I'm just guessing here- if there is other perspectives, I'd love to hear them!