This is a complicated issue.
First of all, radio ratings are mainly done by Arbitron. All stations, commercial and non-commercial, are measured. Only stations that subscribe are actually listed. NPR stations ARE permitted in Arbitron ratings, and are often listed as well. The information is used to demonstrate to donors and funders that their money was well spent.
NPR and PBS were created by federal law in 1967. Their funding was provided through an independent source known as the Corporation For Public Broadcasting. During the past 25 years, the amount of money going to CPB has dropped. Federal money now funds many things, including independent program production, capital equipment purchases, and certain hiring and training programs. But the direct CPB funding of NPR is probably about 2%.
NPR's funding comes from multiple sources. During the NPR financial crisis of 1983, there was a fundamental shift in how NPR got its money. Now, the member radio stations pay for NPR programming through various fees based on market size. The stations get their money from local and state funding, grants, and foundations. NPR also raises money from a wide range of foundations and endowments. There is also the National Endowment for the Arts and the National Endowment for the Humanities, which are funded by the federal government.
Keep in mind that the reason NPR and PBS were created was because it was felt that commercial broadcasting at the time was not measuring up to its potential. This came out of Newton Minnow’s famous “vast wasteland” speech. Commercial broadcasters were driven by market forces that prevented them from providing educational programming. Remove the market forces, meaning the need for ratings to attract advertisers, and the programming would improve. The problem with that is anyone who gives money wants to see results. There’s a lot of competition for financial support, and broadcasters have to demonstrate that people heard the programming. So the use of ratings in public broadcasting is different than commercial broadcasting.
There is a school of thought within non-commercial broadcasting that the ends justify the means. The goal of quality programming, meant to educate and illuminate, justifies the use of commercial tactics such as ratings and even commercials (although with strict guidelines).
Would the programming on non-commercial stations work on commercial stations? Hopefully not. Otherwise, why have non-commercial broadcasting? The purpose is to provide diversity, because not everyone wants the same thing. Those who work in non-commercial broadcasting should aspire to provide programming that appeals to the fringes, more than the mainstream. That’s MY opinion. I can’t promise that everyone in the system agrees. NPR stations frequently have low ratings. It depends on the programming. In Boston, WBUR is one of the highest rated stations in town, mainly because of the news programming. Currently, there is no commercial equivalent to All Things Considered or Morning Edition. The news programming is currently the driving force at NPR, because it attracts the most audience. As a result, NPR has cut back on the amount of non-news programming it does. That’s been the pattern for 25 years. But the more popular the program, the easier it is to attract funding. Just as in the commercial world. Again, do the ends justify the means? As I said, it’s a controversial issue.
But the audience for non-commercial stations is counted, there is no law that excludes NPR stations from participating in ratings, and in some cases NPR stations are attracting more audience than the commercial stations.