There are broader issues at stake here than one man's guilt or innocence. The presumption that one is innocent until proven guilty is the bedrock of justice in our society. One who stands accused of crimes can only be deprived of life or liberty after receiving due process and a fair trial. Our Founders understood the importance of a fair system of justice. American colonists suffered all too often at the hands of kangaroo courts set up by a tyrannical king. The deck was stacked against them and the outcome was often determined in advance of what was little more than a show trial. Therefore, the Founders ensured that a number of judicial safeguards were incorporated into our Bill of Rights, including the rights to due process of law, trial by jury, and a speedy and public trial.
In a just society, criminal prosecutions are hedged in by due process protections. A prosecution should never be pursued with an ends-justifies-the-means mentality. Prosecutors have enormous power. They have the power to destroy the lives of those whom they pursue. It is not the job of the prosecutor just to pursue a conviction. As the Supreme Court has declared, the role of the prosecution is "not that it shall win a case, but that justice shall be done." The merits of the case matter. Vince Lombardi's adage that "winning is the only thing" may apply to football, but it does not apply to criminal prosecutions.
Plato understood the temptation of public servants to abuse the power entrusted to them, asking in his Republic, "Who will guard the guardians?" (Or the more recent iteration, "Who watches the watchmen?") There is no easy answer to this question and no way to guard completely against the abuse of power. Prosecutors are fallible people, and some will inevitably abuse their power. That abuse can, however, be checked by the supervision of judges like Judge Sullivan and the outrage of the public over abuses like the deception in this case.
It is important that people recoil against the prosecutorial abuse that occurred in this case instead of focusing on how Stevens "got away." It is better that one man go unpunished than for an innocent man to be punished for a crime he didn't commit. The safeguards within our judicial system are designed to protect the innocent from the excesses of a powerful government. If we don't utilize those safeguards, we will see far greater injustices than an unpunished gift-grabbing Senator.