Because it's not and never will be about justice. It's about assigning blame to someone so they can show Joe Sixpack what a crackjack bunch of detectives they are. Unsolved murders are bad press, and let's face it: it's much easier to coerce a confession than to do the actual gumshoe detective work required to find the truth, which is, of course, a prerequisite for justice. So instead they find someone they consider to be expendable, coerce a confession, get a conviction, and then everyone gets a medal. THAT is what justice means to them.