The only possible answer is that they're not
Of course the argument goes that NPR is of value because it isn't beholden to commercial interests and pressures. I personally can see some value in that, which is why I send them money during their annual pledge drive. My choice. But for every Conservative taxpayer who's never listened to NPR, the fact they they're basically forced to support public radio though their taxes has got to be frustrating.
Should they be weaned off public funding? Yeah, probably. Not all NPR listeners are in a position to support them financially, but if the ones who are step up to the plate, the federal money would probably never be missed. That's a big "if," though.