Voluntary in the sense that people are working off some "debt" owed to smugglers that can never be repaid.

Chinatown busses are a snake pit of all sorts of crime. Here's an article that scratches the surface. There are numerous sources in the news and academic publications about the links to sex and other human trafficking to these busses.

http://news.wgbh.org/post/human-trafficking-modern-day-slave...