As reported by Rabbi Yair Hoffman in a review, and set of excerpts, of Rishumei Aharon by Rabbi Aharon Felder published by Machon Daas Z’keinim, Lakewood, NJ).
One time, a religious Jew entered the Beis Midrash to speak with Rav Moshe [Feinstein]. He began to relate how his son is sitting in jail on account of dealing drugs. This father requested that Rav Moshe write a letter to the judge on behalf of his son to have mercy upon him. Rav Moshe turned to him and said, “Your son makes people ill and damages them; Let him sit in jail.” Even though the father pleaded and persisted, he refused to budge. He added that his actions were against the laws of the state and he cannot simply do whatever he wants.