There's more explanation in this YouTube video, in which one of the researchers behind the series explains that the goal of the project was to raise awareness in the Netherlands about the historical issue of slavery, and "create attention" about the issue. He also says that the American media "reacted much heavier" to the trailer, "because the topic of slavery has had much more attention."
So it sounds like a culture clash of sorts, with a message meant for Netherlands viewers sneaking out to a much larger, and somewhat more sensitive, audience. The NTR series is set to start up on Dutch TV later on this month, and there is even a "children's version" of the series, called "De Slavernij Junior" starting at the same time.
[Thanks, Jim N]