
China and the USSR were objectively not governed as dictatorships. Stalin and Mao did not constitute the entirety of the decision making power of their entire countries. Also, the U.S has killed far, far more than 60 million civilians, whether through military force (Vietnam, Korea, Afghanistan), regime change (Chile, Guatemala, Venezuela), economic sabotage (any socialist country), diplomatic isolation (Cuba, North Korea).
I somehow doubt the US military is responsible for 60 million civilian deaths. And yes, they did cause those famines. Stalin stealing grain from the Ukrainians for export instead of using it to feed Soviets, Mao ordering farmers to stop farming. That is how one man creates a famine.