It doesn't play into the white men are oppressive and racist and the west is the most horrible place on Earth narrative of the international socialists who seek to tear it down.
As i have said before, Slavery has been a constant in civilisation since civilisation was invented.
The wages of defeat was always slavery or death. What makes the last 150 years or so unusual is actually that slavery has fallen out of fashion (For the time being)
My feeling is that the lack of slavery in the modern world is less to do with social justice and more to do with the fact that the development of the steam engine (And later the internal combustion engine) devalued human muscle power to the point where it no longer had significant economic value and the development of the industrial economy meant that it became easier to exploit the working classes through wages and indebtedness than through chattlehood.
The Greens need to be careful what they wish for. I can easily envisage a world where, should Oil, say, go up to $1000/Bbl and stay there, then slavery would become acceptable once more.