I would assume that somebody who participated in the implementation of a law and then broke it, would be the last person to be able to invoke a good-faith belief that their actions are legal.
However, when your 8 year old daughter who shared an Miley Bieber mp3 had good faith belief she was ok because everyone is doing -- that is untrustworthy, and the fact that you have good faith belief in her intentions does not clear you of anything. You'll go to jail or pay a hefty sum.
That's how laws really work. Not the way they common knowledge assumes.