"To be precise: the United States government cannot silence the pope, and it cannot silence any other religious leader."
Actually, it can. Popes have been "silenced" in the past. The best known example was Napoleon's general arresting, in 1809, Pius VI, holding him until he died in captivity. Clement VII was captured by the troops of Emperor Charles V in 1527, required to pay a large ransom for his life. Henry IV attacked Rome in ~1081 and drove the pope from his throne.
Popes are not invincible. As heads of state, they are legitimate military targets.