We (in the US) had the Union of Soviet Socialist Republics as our main adversary for a long time, justifiably or not. Considering the levels that rhetoric reached between the early 1920s and 1991 (with a brief break for WW2), it really isn't surprising that socialism and communism came to mean the same thing in the minds of many Americans, and that anyone who was advocating a rational socialist system (e.g., Pete Seeger) was seen as a Soviet sympathizer. Not that the Soviets ever actually practiced communism, of course.