Why are there people in the US that believe communism and fascism are the same thing?

There are people in America who believe that every ideology that ever existed is Communism if it doesn't agree with their own. The worst part is not the unflinching ignorance, but their jaw-dropping arrogance on the matter. "Duh, Nazi has the word 'socialist' in it, therefore Stalin was a Nazi who was Che Guevara who started Prohibition which invented the British Monarchy which is Satan who is a freeloading welfare queen who votes Democrats. Haha! Another lib smashed!"