They say if the witnesses were so scared, that they had their own free will to just leave or call the police. That's not how fear works. These girls saw what happened if they stepped out of line. Where would they even go if they did escape? If this is their first time in the city from the village, and being young, they will have no idea of where to go and who to ask for help. And being prostitutes, they most likely won't go to the police. 
Just as in domestic abuse; you can't tell a woman to 'just leave' an abusive partner, as smart as she may be. Fear keeps her home, in the same way as these witnesses' fear keeps them from running too.