The ban on listening had to be a bit more complicated.
https://en.wikipedia.org/wiki/World_War_II_Radio_Heroes
At first , the NY Times listed the times of Axis broadcast of POW messages, and some in the US who had radios with SW bands forwarded the messages by mail to US families. Eventually the government decried the practice and went after a few practitioners. Of course, they could not actually stop listening without a program of confiscating radios, but could repress the spreading of the likely false info.