In the American criminal justice system, defendants are entitled to the presumption of innocence. This is often stated by saying that one is considered "innocent until proven guilty." In order to convict the defendant, the prosecution has the burden of proof. The defendant does not have to speak or otherwise give testimony that could be damaging to their defense. It is up to the prosecution to prove the case.
The prosecution is in a somewhat unique situation in that on the one hand their job is to prosecute and convict the guilty, they are also supposed to let the innocent go free. However, like any job, there is pressure for a prosecutor to win. For years members of the defense bar and criminal justice experts have said that due to prosecutor misconduct, the balance is tipped too far in favor of the prosecution as prosecutors are cheating the rules to win convictions. A recent USA Today piece on the subject supports that conclusion and sheds new light on the fairness of the criminal justice system. It's not pretty.
According to the article, in many cases, federal prosecutors kept key exculpatory facts from the jury and the defendant. Prosecutors covered up evidence in a number of ways, failed to disclose the deals that they had promised convicts in exchange for their cooperation, broke plea bargain deals and lied to judges and juries. The cheating has come at a huge cost. USA Today reports that prosecutorial misconduct has cost millions of dollars in legal fees, landed innocent people in prison and set guilty people free.
It's an excellent article and it far more in depth than we have time to go into. Looking for a takeaway? Innocent or not, it's best to avoid all contact with the criminal justice system. You are entitled to the presumption of innocence, but win or lose, it's a long, tiring, expensive process and as the investigation demonstrates, it's far from perfect.