There are some there that have no choice and are trapped. You don't need chains to keep someone held hostage or make them do something they really would prefer not to do...in fact most victims of human trafficking are not actually chained down. It is usually a financial situation or they are far away from home and can't afford to go home. Many of the girls there are from poor provinces and have no way of getting home if they just walk out. The threat of being homeless and hungry can make people do some stuff they normally wouldn't. And do you really think running away to the police is going to do them any good? The Philippine government is not really serious about stopping human trafficking and wouldn't put out a peso to help a girl who wants to go home from Angeles. (Not unless there is some money to be made from a business owner or they can get some good press from it.)
However, most of the busts you hear about in AC are because the owners didn't pay off the right people for a couple months and once the girls are released from their "rescuers" they will be right back in the bar.