The problem in the countries that generally allow prostitution is that, though it has been legalized, it is still horribly stigmatized. Which creates that gap where forced prostitution can flourish. If a "business" is using illegal immigrants as forced labor, who can they turn to for help? That's a double dose of stigma right there. But even a native person who's being coerced would likely get little in the way of help since "upstanding" society looks down on their chosen profession.

Our collective worldly society needs to stop looking down their nose at sex workers as some kind of deviancy.

But then, seeing how religious extremists seem to be gaining power all over, I doubt we'll see any positive change on any of these things.