In America I generally trust that the FBI will have no mercy when cracking down on human trafficking. I also trust that the vast majority of sex workers are legit, consenting people who consciously chose to do it.
I don't know much about Vietnam, but I am guessing police/FBI-esque corruption is much higher than America/Europe which is why I would also guess that this accounts for the larger percentage of sex workers there that are trafficked.
https://en.m.wikipedia.org/wiki/Corruption_Perceptions_Index
How can you say "just because"? Several other reasons were plainly listed?
I might also note that I lived in southeast Asia for many years. There's a huge amount of trafficking that isn't being reported to the Atlantic website or the US State department there.
If you'd review the guidelines at https://news.ycombinator.com/newsguidelines.html and follow them, we'd appreciate it. This one is particularly relevant: "Please respond to the strongest plausible interpretation of what someone says, not a weaker one that's easier to criticize. Assume good faith."