They think the worst. Flashbacks of the USSR and diving under desks to protect against a nuclear threat in a cold war period. Vietnam, China, North Korea, Cuba, all bad "communist" states. When people say communist, they mean it in a bad way, when the real definition does not suggest whatever they are meaning. Communism has been the propaganda rally for leaders such as Lenin, Stalin, and Castro, that the word has taken on a bad meaning, and thusly, is thought to be evil. Because of this, it is often used to insult political leaders, by labeling their actions as "communist", or "socialist", in order to sway supporters.
It's all a bunch of crap really, based upon the fact that people are so ignorant.