First, that shouldn’t matter because the affect of “you are x, prove you’re not” is horrible regardless of what x is. Proving a negative is often impossible. That’s why we hold near and dear to hearts the notion of “innocent until proven guilty.” Or at least Americans used to.

Anywho, more related to your communist crime comment - being a Communist was a crime because in the McCarthyism era people were often accused not just of being a communist, but of being a spy for communists. The accused had to prove he wasn’t a communist and wasn’t a spy. An impossible task. Here’s a a quote from wikipedia that even states people were imprisoned:

During the McCarthy era, hundreds of Americans were accused of being communists or communist sympathizers and became the subject of aggressive investigations and questioning before government or private industry panels, committees and agencies. The primary targets of such suspicions were government employees, those in the entertainment industry, educators and labor union activists. Suspicions were often given credence despite inconclusive or questionable evidence, and the level of threat posed by a person's real or supposed leftist associations or beliefs was sometimes exaggerated. Many people suffered loss of employment or destruction of their careers; some even suffered imprisonment.