Do I have to take my turn and produce a radio show? Surely not. We can't be lassoed into obligations that easily--just because some group of people decides to jointly bring about a public benefit.
My thought is: OK, I agree with Nozick about that scenario--you don't have to produce the radio show on your appointed day. But what's going on here, and how is this different from other cases in which you are a nefarious freerider for refusing to contribute to a joint enterprise you benefit from but didn't request or initiate?