She was not. Both of the Rosenburgs recommended Ruth Greenglass as agent, and for a recommendation to be accepted from you, you had to be a trusted comrade. Moscow wanted confirmation of Ethel's trusted status and they got it. Their "rezidentura" vouched for her and listed her level of knowledge about her hubby's activities. She knew about what her husband was doing for them and knew about his contacts. The confirmation of her loyalty and value was signed by Leonid Kvasnikov, a soviet intelligence officer at the time. He was the guy who was in charge of nuclear/atomic spying. Ethel knew what was going on, she was in a position to actively support with Greenglass.
"Of course they were guilty. but you can't quote me. My public position is that the Rosenburgs were innocent. ...What's wrong with what they did/ If I were in their place, I would have done the same thing...It was the responsibility of a good Communist to do whatever he could to help the Red Army gain victory. Don't think i'm so dumb that I don't believe the Rosenburgs weren't engaged in espionage." - lawyer Victor Rabinowitz, first guy approached by Julius to be his counsel, as told to former communist Ronald Radosh
This is good enough for a whole other thread. You may be right, but from what I've read, including a later recantation from a star witness shows me she was innocent. The FBI jailed her to put pressure on Julius to confess and give up the whole spy ring, and when he didn't spill everything, they had no choice but to proceed with her prosecution or they'd look bad.