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This is true, and it connects to the fact that drone strikes do not involve American casualties. You can't bring the troops home if they're drone operators already based in the U.S.. But there's another reason for the focus on drones. Under the last two administrations, drones have become a ubiquitous tool of military intervention in countries that the U.S. is not even said to be at war with.
If anti-war activists go to Americans and throw out slogans like: End the War in Yemen! Most people won't know what they're talking about. Everyone understood about the war in Iraq and understands about the Afghan war. But, is the U.S. at war with Pakistan, Yemen, Somalia, post-Qaddafi Libya, Mali and who knows what other nations that drones are prowling the skies of? Perhaps, but with the exception of Pakistan these are conflicts even more subterranean than some U.S. covert interventions during the Cold War.
Finally, anti-drone rhetoric may be popular for one more reason: drone strikes, in the way that they pierce the veil of civilian life with explosive violence, resemble the bombings of Islamic militants that decades of propaganda have branded terrorism. Thus, criticizing drones is a way of calling the U.S. a terrorist state.