What surprises me is that Netherland, while praised for doing the most to fight slavery, is, at #50, ranked lower than the US, which I believe has forced prison labour.

Maybe it has to do with prostitution being legal here. You'd expect licensed and regulated prostitution to reduce slavery in prostitution, but apparently it's still a serious problem. Still, 17,500 people in slavery? That's a lot. I'd like to know where they got that figure.