Congress has nothing to say about it. NPR is a private company with no connection to congress.
All of these stations are independently owned. NPR doesn't force anyone to carry their programming. It all decided locally. The stations pay based on the amount of programming they carry.
What I was referring to was not recent. At the time that WETA went from all-news to all-classical in 2007, I remember (but can't find now) hearing reports that some Congress members were angry that some markets (specifically Washington, D.C.) had two NPR news outlets. I now believe that that anger was specifically aimed at the WAMU/WETA situation in the D.C. area between 2005 and 2007. What would have happened if Congress had passed a law at that time mandating the number of NPR news outlets in a single market would have been up to the courts.