The effect of one man and, more precisely, one man's wealth and fame on popular perceptions of the American brand of criminal justice is immeasurable. Of course I am referring to the original O.J. Simpson case. So many people who watched the case unfold more than 10 years ago feel that they learned one or more things about how the system functions, or malfunctions... The "lesson" cited most often is that guilty people sometimes go free; particulary wealthy guilty people.
I recall OJ making his first appearance on homicide charges where he declared that he was "Absolutely, positively, 100 percent, not guilty". I don't know who counseled him to modify the plea of "not guilty" but, if anyone did, it was more likely a publicist than a lawyer. "Not guilty" is one of only six permissible pleas to one or more criminal charges in California. Others are: guilty, nolo contendere, prior conviction, once in jeopardy and not guilty by reason of insanity. There is no legally cognizable plea of "Absolutely, not guilty" much less "Absolutely, Positively, 100 percent, not guilty". In all likelihood, OJ said it with so much emphasis because he knew the whole world of mostly non-lawyers was watching, and there is something about saying "not guilty" that just sounds, well, guilty.
The truth is, any time someone sets foot in a courtroom, as a criminal defendant, nearly every spectator and participant assumes that there must be some truth to the charge(s), otherwise the person would not be charged. So, saying "not guilty", however emphatic, lacks the persuasive force it would otherwise have. If we, as a society, weren't already conditioned to believe that nearly everyone charged is guilty, then we might actually listen with interest to what the plea is. What is not as widely understood is that nearly everyone who is criminally charged pleads "not guilty" - or at least authorizes an attorney to say it for them - at the beginning of the case. And there is a reason for this. Saying the words "not guilty" is the only legally recognized way to express the four most common states of mind held by a charged individual, which are: (1) "I am innocent",(2) "though I might be guilty, the state can't prove me guilty", (3)"I am guilty of some, but not all of the charges", or (4) "I am guilty as charged but am not ready to make a deal with the prosecutor right now". The lack of readiness may be because the charged individual wants to know more about the strength of the case against him and/or what the punishment will be, and whether any deal offered as of yet is the best deal he can get. So, "not guilty" is not a lie in cases where the person is actually guilty.
In traffic court you will hear people say they are "guilty with an explanation". These are, however, guilty pleas where the court is allowing the individual to blend their guilty plea with their argument for a lenient sentence. Such "pleas" are not permitted in non-traffic criminal court, and never result in anything other than a conviction of the charged offense.
To make matters more complex, there is a type of plea that permits an assertion of innocence along with a plea of nolo contendere. Such pleas in California are made pursuant to a case called the People v. West (1970) 3 Cal.3d 595. A "West plea" is most likely to stand for a plea which "does not constitute an express admission of guilt but only a consent to be punished as if guilty." Plea bargaining, which is often analogized with sausage-making, leads to many fictions and, at the fringes, injustices both large and small. A reality of our, and perhaps any, justice system is that people occasionally plead guilty to charges they didn't commit, and plead guilty to having done something criminal even where they haven't done anything criminal. This is what comes about by way of the immense legal leverage the legislature, and the voting public, has given to our prosecutors. It happens that people who are innocent sometimes agree to plead to a charge in order to eliminate a risk of a particular undesirable outcome (e.g., lengthy incarceration).
The most useful way to think about a criminal case, if you are the person charged, is that it is, at its core, a business transaction. The state wants one or more things from you and what you want in return is fairness. In many cases, the only way to achieve fairness in return for your eventual plea of "guilty" or "nolo contendere" is to, at first, plead "Not Guilty".