A lot of employers have this. This isn't that strange. You might have the same. I've had to run open source work past employers when it's similar to the company's domain.
Right, but NPR isn't any old employer. It was created by Congress with the Public Broadcasting Act of 1967 and is a 501(c)(3) nonprofit with a mission of "creating a more informed public, one that is challenged and invigorated by a deeper understanding and appreciation of events, ideas, and cultures." Despite it not receiving that much taxpayer funding, I would hold it to a standard of a government organization; and I expect diverse viewpoints and dissent to be a core part of that mission.