Hidden messages??? Subliminal messages???

are hidden messages, backmasking, subliminal messages all done on purpose? I mean their are songs like: Queen- Another one bites the dust (and if you play the chorus backwards, it becomes It's fun to smoke marijuana). Another example would be: Daft Punk's song called, harder, better, faster, stronger.... which if played backwards it becomes a hidden message about a girl called erva and some pretty nasty stuff!!! So my question is : Do The Artists That Record A Song Containing A Hidden or Subliminal Message Do It On Purpose.. Or Is It All Some Mistake????