They get 50% of their budget from taxes, and most of the "tasteful sponsorships" are large companies that either get large subsidies from the overnment, or work under lots of government regulation and want to buy favor with congress.
The largest sponsor of NPR is Archer Daniels Midland, an agricultural corporation. Want to know why Scientific American is talking about how food shortages could kill tens of millions over the next few years?
http://www.sciam.com/article.cfm?id=civilization-food-shorta...
It's because ADM has been using its NPR sponsorship to buy access in Washington, where it can push biofuel mandates, turning food into a poor substitute for gasoline, thus boosting the price of it beyond the reach of the global poor.
Possibly there would be problems tracking downloads for sponsors. Or, maybe it's the structure of how pledge drives work, where a breakout show like TAL helps fund the entire station.
They are the one traditional news outlet that isn't damaged by changes in distribution methods. The only change might be that they need to spread their pledge drives out more broadly so that it's harder to skip or tune out.