"There's no excuse for leaving the Church...."
He didn't leave "The Church", P, just the particular Latin Church. I had thought by now that +BXVI's definition of The Church, the particular churches within The Church and "eccelesial assemblies" would have made it down to the level of the laity.
What he left was specifically the secular AmChurch.
Sure he did. He's renounced the dogma of Papal primacy and Infallibility. That makes him a heretic, and heretics cannot be members of the Church, since the members of the Church have "one faith".
We can speak of the Orthodox as being a quasi-communion with the Catholic Church because the division between us as laity is not a result of either of us changing our faith to something different from what we understand the Apostles to have handed down, but over disagreements between our Bishops over the nature of that faith and how it impacts the organization and operation of the Church. Given such a situation, it is expected that there are material differences between us in certain points of the faith, however these differences are merely material, and not formal, because there is a lack of willfullness in the differences.
That being said, Communion in the Church is a one way affair that flows down from the Pope through the Bishops to the laity. We are in the Church because we hold the faith and are in communion with our Bishop and through our Bishop with the Pope. We are not in the Church because we simply proclaim ourselves so to be. The quasi-Communion we believe to hold with the Orthodox flows from tacit Papal recognition/granting of spiritual jurisdiction to the Bishops of the East (i.e. he has not and does not excommunicate them, unlike the renegades among the Chinese or the Old Catholics or the SSPX, but instead actually recognizes their spiritual authority and holds it to be valid). However, despite this, it is quite possible for a Catholic to take up heresy, as Rod Dreher has done, leave the Church, and attempt to affiliate with the Orthodox. Such a plan of action does not leave one remaining inside the Church since it involves formal rejection of communion with the See of Peter.
This is quite different from a Protestant becoming Orthodox, or an Orthodox simply being himself, because their the acceptance of the Orthodox faith involves a tacit admission of some form of Papal jurisdiction and communion, since the history of Orthodoxy does include at least some recognition of the Roman claims, rather than an outright rejection of it which leaving the Catholic Church necessarily involves.
There's a grand canyon between an Eastern "rite" Catholic Church (Armenian, Byzantine, Chaldean, Maronite, etc.) and Eastern orthodox - it's called the "primacy of Peter". Dreher stated that he cannot accept this. So be it. But to say he's making a lateral move is nonsense when he is removing himself from the flock of Peter.