As the prosecution prepares its case, it does find evidence that corroborates the defendants' stories, but they choose to hide that fact from the defense. Again, in the movie, when the prosecution wins the case initially, you can see the law-makers getting great pleasure out of this. Most people who accomplish something usually are of the type that they form an opinion about something and it takes a lot to shake that opinion. We see this happen constantly, even in the scientific world where researchers push their pet theories. It is these sort of people that we have in in the legal systems around the world.
Some systems are worse than others, but the basic problem is the same.
So given this type of people in our judicial and governmental, we are always going to have cases of innocent people being persecuted. The system will be proportionally twisted to the degree of corruption in a society. My personal philosophy is to do away with any sort of system that gives so much power to a set of people that the individual is virtually powerless. To take things to an extreme, I believe in an anarchistic society where people can do what they want and people can be their own judge and jury. I think the system of justice does punish them, but I don't think it really makes a big dent in reforming the criminal mind.