Aren't NY, CT, MA, and DC the most unequal parts of the US? All "Union"? I don't think your explanation is convincing.
Edit: the arstechnica article states DC is the most unequal place in the U.S. It appears there is a disparity between the two sources.
That map is by county, while most inequality in the US is concentrated in the cities; the most important drivers of state-level inequality are thus hidden. It is not a very good map for this purpose.
I can agree that neither map tells the whole story, and the GP's premise is strained at best.