You can't just call 1.3 billion people and their forbears over 500 years "heretics" and anathematize them. "Heretic" would have little meaning. A heretic is one who works to effect doctrinal schism in the Church, and that requires intent and immediacy. A heretic's children are not anathematized. Nor his great-grandchildren. He is. How much less the great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great (I think that's the right number of "great"s)-grandchildren of those who were simply born into Christian Europe on the wrong side of a geographico-political line at the wrong time ...
You are conflating the 'heretic' part and the 'anathema' part.
The founder or leader of a heretical movement is called a heresiarch, while individuals who espouse heresy or commit heresy are known as heretics.
Continuing along in heretical religion, no matter how far removed from the founder, still makes one a heretic.
Not all heretics are personally anathemetized for their heresy.....since most of them are not -in the Church- to begin with and thus the first sentence of the following would be pointless.
"The two causes for which a person may be anathematized are heresy and schism. Anathematization is only a last resort, and must always be preceded by pastoral attempts to reason with the offender and bring about their restoration.
For the Orthodox, anathema is not final damnation. God alone is the judge of the living and the dead, and up until the moment of death repentance is always possible. The purpose of public anathema is twofold: to warn the one condemned and bring about his repentance, and to warn others away from his error. Everything is done for the purpose of the salvation of souls. " wikipedia...yea...shoot me for bad references..
So while there may be an anathema on a particular heresy, a decedent of the original heretical group, who is just following along with what they have been taught is still in heresy. Just unknowing heresy.
If anything, finding the Church and being restored (via Baptism, etc.) would be the goal for these decedents in heresy.
please note, I speak from personal experience here...my ancestors were heretics against the Church, my relatives are still heretics, and I was one until I renounced any shared beliefs with them and was received into the Church. My ignorance and inheritance of heretical beliefs did not make me not a heretic.