This is probably a stupid question, but WHY is communist organization allowed to operate in the U.S.???
Don’t know. They are an official political party who usually run members for the presidency. They apparently didn’t feel the need to run anyone in 2008. Workers World Party is another legal communist party here in the US. The Revolutionary Communist Party, however, is not a legal political party.
They’ve been operating as the Democrat party for years now.
There are really two answers to this: The more or less official one and the truth. The "official" one is that the same law must apply to all and communist organizations could not be suppressed without constitutional and legal changes that could be later used against other, more legitimate, organizations.
However, since the CPUSA has openly declared its criminal intent (mass theft, suppression of free speech, racial discrimination, ad nauseum), its protection under the First Amendment is at least questionable.
The truth, therefore, is that while the CPUSA itself is small and unpopular; its talking points, beliefs, and philosophy are pervasive among the most powerful elements of this society. These include the media and artistic communities, the academic community, organized labor, the unpublicized but hugely influential religious left, and many other self-perceived elitist elements. This can be shown by comparing liberal and media memes to the CPUSA's talking points and FAQ, which closely resemble the boilerplate you are likely to hear every day from rank and file libs or community activists. As a result of this pervasive influence, anyone who demands real action against communist traitors is automatically demonized and shouted down as a McCarthyite, a fascist, or an uncouth tea-partier.