We experienced this phenomenon throughout the Cold War. U.S. officials were spying on Americans who believed in communism or socialism, tapping their telephones, keeping secret files on them, infiltrating their organizations, and doing whatever they could to destroy, humiliate, or ruin them.
Why were they doing such things? Because they believed that Americans who believed in communism or socialism were subversives and even traitors.
It was the same mindset during the Vietnam War, when U.S. officials secretly monitored the activities of American citizens who were protesting the war. Anyone who wasn't "supporting the troops" was considered a subversive, a bad guy, a traitor. Anyone who refused a conscription order to go fight in Vietnam was criminally prosecuted as a draft resister.