Will Wilkinson has an interesting post up arguing that Rawls is inconsistent in the idealizing assumptions he makes in A Theory of Justice. The gist of it is that Rawls initially says (in §2) that he is assuming “strict compliance” (i.e. everyone is assumed to have an effective sense of justice), leaving problems of “partial compliance” for later (e.g. just war, rebellion, civil disobedience), but then (in §42) admits that we need the coercive power of the state to ensure that people don’t free ride, which seems inconsistent.
I don’t think Rawls is retracting his strict compliance assumption, in Section 42. It’s just that he assumes that the sense of justice involves a reciprocity condition. If most other people are complying with just rules, I will too, even if I think I could get away with not doing so, but I am not willing to comply with just rules if most other people are ignoring them. Strict compliance means that everyone has a sense of justice and this fact is public knowledge, not that all duties of justice are binding unilaterally. In Section 42, I think Rawls is just relaxing the common knowledge part of strict compliance. If everyone was 100% certain that everyone was properly motivated by their sense of justice, there would be no assurance problem, even though the sense of justice involves a reciprocity condition. But if there are any doubts about the motivations of others, cooperation could break down, even if it were true that everyone was in fact motivated by justice.
The connection between justice and reciprocity explains other things in Rawls, I think, such as how duties of distributive justice can be global in reach, in one respect, but limited to fellow citizens, in another. Wherever we interact with other people, we have a duty to help create just institutions, to regulate this interaction. We also have a duty to aid those in distress. These duties are binding unilaterally, but they are also not very demanding (in Rawls’s view; the duty to create just institutions is on p.115 and 334). The duty to share fairly in the benefits of cooperation is demanding, but it is binding only where there is a reasonable assurance of reciprocity, i.e. where institutions are in place.