Commentary for Avodah Zarah 82:2

R. Yohanan says that since no idolater actively annulled the idol, the fragments are still considered idolatrous and they are prohibited. R. Shimon b. Lakish tries to get into the head of the idolater. If the idolater sees that his idol broke, he will divest it of its power, thinking to himself that if the idol could not save itself from being broken, then how can it save me? Thus we can assume that it was annulled and its fragments are permitted.