The point is that it skirts the law. You can’t really make it illegal because it is a way of subverting legality. If they legally obtain the evidence then it’s legally obtained. If they happened to get to that point through extra-legal means that doesn’t really matter, as long as the end result is legal. Maybe you could argue in court that they only got there because of extra-legal actions, but they can argue the opposite. If this helps them look in the right spot for illegal actions, who’s to say that them looking there couldn’t have happened purely by chance?