Offshore pirate radio stations were wildly popular in the 60s, they were considered illegal because they didn't have permission to broadcast the content. They couldn't get a license so they went offshore. What made the content illegal wasn't clear, it was somewhat of a legal grey area that allowed for the governments to do whatever they wanted.
The governments wanted control so they were fighting it with legislation, they jammed the broadcasts, they made it illegal to receive the content and they raided the artificial islands. I'm not sure whether the government raided the ships as well.