I have a strong feeling that they will charge for it eventually. Maybe the app will ask them to donate as they use it?
For example, if Planet Money produced a monthly bonus episode for, say, $5, I know I wouldn't think twice before tapping "Buy".
At least, that's how I would do it. NPR is missing out on a big donation opportunity by not taking advantage of an app store's built-in in-app purchase/donation infrastructure.