As a native speaker of American English, I'd say that neither ordering is incorrect. Your suggestion ("illegal everywhere") is indeed more common, and would usually be better choice in conversational speech and popular writing. The subtitle ("everywhere illegal") isn't wrong, though, just stilted. Presuming the author is a native speaker, it's almost certainly not an error, but a conscious choice to employ "hyperbaton".
Overused, this sort of inverted word order can leave you sounding like a mishmash of Monty Python, Yoda, and a bad fantasy novel, but applied selectively, it acts as a defamiliarization that adds emphasis. Here, the inverted ordering conveys a sense of being legalistic, Biblical, and archaic. Presumably this is intended to hint to the reader that piece will argue that the practice of slavery is similarly out-of-place in the modern world.
I also think this added emphasis. I liked the way the author phrased it, i.e. "...everywhere illegal". To me, it gave the impression of focusing on the word "everywhere". (I'm not a native speaker.)
Overused, this sort of inverted word order can leave you sounding like a mishmash of Monty Python, Yoda, and a bad fantasy novel, but applied selectively, it acts as a defamiliarization that adds emphasis. Here, the inverted ordering conveys a sense of being legalistic, Biblical, and archaic. Presumably this is intended to hint to the reader that piece will argue that the practice of slavery is similarly out-of-place in the modern world.