Ok Justin, let's ask you a question.
Let's say you are a poor person of a visible minority in 1975. Let's say someone close to you gets murdered in the home you live in like a girlfriend,( I do not wish this upon you, this is just hypothetical). Let's say they charge you with the murder, some witnesses say they saw you arguing with the deceased, your physical evidence is all over the place and on the body of the deceased like hair samples and your blood under her fingernails, the cop is a racist who treats you like shit and you aren't too smart due to a learning disability which teachers failed to recognize so you aren't to sure about your rights, you don't have a lawyer except for a public defender who doesn't fight too hard for you as they are probably overwhelmed with tons of other cases, an all white jury convicts you of the murder based on the evidence despite the fact you didn't do it because while you went for a walk to clear your head after the fight, someone broke into your home and killed the deceased cause she caught them trying to steal your tv, they panicked and left with nothing. They sentence you to death. You die after 15 years of appeals. Then in 1995, DNA evidence proves you didn't do it, it was another guy they already have convicted for 2 other murders. How do you feel about it now? You're dead though. Ruben Carter had people who believed in him as did many others, this may not be the case with Tookie but it is in many other cases.
The police in many cities and the DA are desperate to pin a murder on people. Society feels safer when someone goes to jail for murder. Hence, all the bull that happens during the whole prosecution process. Unfortunately, there are hundreds of murderers walking around free in our society who will never be caught.
And as Bonovox said, we don't want the murderers to go free, clemency or commutation is not freedom. They spend the rest of their lives incarcerated, locked away from society with no freedom.