Those member stations choose to spend that money on NPR content. They are under no obligation (aside from standing contracts) to continue to give that money to NPR in the future. They could opt to license programming from other sources.
That's like saying a federal employee who chooses to donate some of their income to NPR is giving them government funding.
I agree to some extent, but when the federal government is funding similar organizations with a similar mission, they have a strong incentive to work together. Public radio stations are operated and listened to by the kind of people who like NPR programming.
It's more like a federal employee who works for NPR and believes in NPR's mission donating to NPR. Yes, it's not compulsory, but the money is flowing towards entities with closely related missions that have a strong incentive to cooperate.
But again, I agree with you that the voluntary nature of the cooperation makes it quite different from merely some kind of shell game.
That's like saying a federal employee who chooses to donate some of their income to NPR is giving them government funding.