A whole lot of people get really rich heading up or working for non-profits.
I'd say "heading up" more than "working for". And I think the article is right to focus on the disparities.
Non-profit is also a major plank of liberal American culture that really got off the ground after the culture wars of the 1980's put the final nail in the coffin of tax payer funded cultural organizations. Since then it's developed into a massive money conduit with its own language and strategies. In the case of nyc theaters like Roundabout, the non-profit structure allowed the organization to amass quite a large real estate portfolio.