I suspect it has to do with not understanding communism in any of its forms. Even the most educated of the Tea Party fail in their definitions, fail in their interpretations of our Bill of Rights, fail in their interpretations of our Constitution, fail in their identification of our form of government (Democratic Republic).
Realistically much of the Tea Party leadership is playing to the crowd. They are using language to inflame their constituents. The evil of Communism is much easier for many to understand and when combined with Nazism, even better to keep the masses riled up. This combination means "godless", it means loss of freedom, it means loss of personal property. These are things Americans think they understand and will fight against. Will rally against. Will vote against even to the point of voting against their own best interest as we saw in the mid-term elections.
I think junkseller has it right, the Tea Party isn't going to be inconvenienced with real definitions or truth. So long as they can throw names, point fingers and cause enough people to fear we will be a communist country, the POTUS will be a Communist or a Muslim or not a Citizen, simply because they say so.