It's because in the US justice system you are considered innocent until proven guilty. "Proof" requires an adversarial test to ensure you're not only getting on

It's because in the US justice system you are considered innocent until proven guilty. "Proof" requires an adversarial test to ensure you're not only getting one side of the story. Without defense attorneys, nobody could be "proven" guilty because a show trial proves nothing. If you want someone to be eligible for punishment, you should thus WANT them to be defended.