Originally Posted By: SC

I repeat - Vito Corleone or any of his sons would NEVER EVER go to a court for justice.

But Tom was a lawyer. He even threatens McCluskey that he would have to show cause in a court of law. no one except Michael thought that they could kill a policeman. So whatever Vito's private justice system was, he or his men weren't ready to handle McCluskey outside the court. It was unprecedented, until Michael comes up with his bold plan.