Benedict's letter on the SSPX excommunications
The pope's letter acknowledges that the excommunication remissions came as a surprise to all and were issued without adequate contemporaneous explanations. As a result, even friends of the pope were ill-prepared to defend his actions in the court of human opinion (and yes, the court of human opinion matters, as evidenced by the pope's letter itself). Church enemies had a field day amid the confusion, but since that's what enemies do anyway, there's not much more to be said about them.
In any case, the canonical explanations I suggested for the excommunication remission on 27 Jan 2009 (see also 24 Jan 2009 and even 3 Feb 2006) now seem verified. I had said that: either the pope was acting on the basis of information available to him privately concerning the necessary withdrawal from contumacy before censures can be lifted under 1983 CIC 1358, or, that the pope was acting praeter legem in lifting the censures as a pure gesture of mercy.
Now Benedict writes: "The withdrawal of the excommunication . . . was possible after the affected had expressed their fundamental recognition of the pope and his pastoral authority, albeit with reservations as far as obedience to his magisterial authority and that of the Council is concerned." I take this to mean that the four bishops expressed to Benedict their regret at having taken episcopal orders contrary to the directives of John Paul II (albeit with some canonically irrelevant reservations about popes and councils). If I am right about that, such an apology would suffice as a withdrawal from contumacy for the act of illegal ordinations and would enable Benedict XVI to lift the penalty in accord with Canon 1358. But, even if the bishops did not apologize for their action, the pope (but no other authority in the Church) could still have lifted the excommunications as a praeter legem gesture of mercy intended to encourage the SSPX to move toward full communion. If the pope's action were of the second sort, one might be able to question its prudence (c. 212), but not its legality.
Briefly, in regard to two other points: (1) to no one's surprise (well, not if they read In Light of the Law!), Benedict XVI did not repudiate John Paul II's imposition of the excommunication in 1988; and (2) the pope confirmed that the remission of the censures impacts only the canonical status of four individual bishops and has no impact on the status of the SSPX itself, which is still irregular.