This is my thinking. They don't have enough power and influence yet to just come out and say "Yeah we're communists, so what?" so they need something to smear or discredit people who are on to them.

"Anti-Semitism" works at least some of the time, so it's valuable to them. Interpreting this as "That proves it! There's a Jewish conspiracy!" is a huge mistake. Plays right into their hands.