The same way the Eastern Orthodox churches can still have validly ordained clergy, and valid sacraments.
What it comes down to is that, in spite of diobedience to the Magisterium of the Church, Apostolic Succession can still be preserved through the Sacrament of Holy Orders. A Bishop who breaks with the Church is still a Bishop, and when he ordains, with the proper form and intent, the ordination is sacramentally valid, though illicit.
Remember too, excommunication is something that someone inflicts upon themselves – they separate themselves from the communion of the Church. The formal pronouncements of excommunication by the Church formally and publicly declare that fact (which remains a fact, even without formal declaration), either for the sake of the soul of the individual, the souls of the faithful, or both.
Blessings,
Gerry
