Is the majority of these populations truly atheists, or just the leaders/state belief? A belief in God, really doesn't go against the teachings of Buddha.
Judging from the reinstatement of the Church in Russia, it seems the atheists had only temporary control. In China there is a large population of theists, although the godless communists still have power. The Buddhists, I don't know, except their cosmology doesn't seem to involve a Creator.