Instead, I think it's better to consider what is "natural" for certain societies and cultures. For example, most people in average, relatively prosperous Western countries (and many Eastern ones) would not "naturally" treat another human being the way the "clients" in the article do. A few sociopaths, but that's a very small percentage of the population.
Why? I think it's pretty simple: it's how people are raised. Children who are raised by loving parents, taught to treat others well, disciplined when they don't, grow up to be adults who wouldn't dream of doing such a thing. But a child who is raised by abusive parents, who witnesses the parents treating others like animals, may grow up and emulate that behavior. If they are taught that it's acceptable to treat other humans like animals and property, then that is what they will do.
Nature vs. nurture is a bit of a false dichotomy. No one grows up in a vacuum. People are shaped by family, society, culture. People tend to emulate what they see. Getting people to change their behavior and values by reasoning with them is very difficult, and impossible with some people.
This is an element that seems sorely lacking in the article: there's no mention of cultural and societal values. She mentions corruption in police and judicial systems, and she calls for unilateral, cross-border action--which would effectively be going to war. She doesn't recognize that you can't fix those problems by going to war, because war doesn't rewrite societal values. (On the other hand, one might point to Nazi Germany and Imperial Japan and post-WWII as counter-examples, but I think that is a bit different: it was a world war, and the nations were occupied for many years afterwards, and a government had arose which lead the people in evil ways. The cultures weren't necessarily innately evil in their values.)
The reason human trafficking flourishes in these places and police and judicial corruption allow it to continue is that the societal values are corrupt. There's an element of oppressive governments not accurately reflecting the values of the people as well, but still, the government isn't forcing these atrocities to occur.
And to bring it back to where we started: by defining "natural" by what seems natural to animals, you perpetuate the problem. Animals have no concept of right and wrong. Human trafficking and slavery are wrong because they are wrong, because they are evil. You might say that that's circular reasoning, but if we agree on that basic morality, regardless of where it comes from, then it stands.
And therefore, the reason these other places continue to allow it is because of defective moral values in a large percentage of the population.
That isn't a popular thing to say, because it's "judgmental." Well, yes, I do judge that that behavior is abhorrent, and that the people engaging in it and permitting it are acting abhorrently.
And if you want to argue that it's not a result of defective cultural values, you'll have to do better than an "animal nature"-based argument, because that would logically erase cultural differences. And, no, a few outliers in cultures that largely don't have the same problem do not wash away the distinctions. Some cultures do have better moral values than others. (If you disagree with this, you're a moral relativist, and there's no basis for discussing it.)