The prosecution prosecutes people that they BELIEVE are guilty....they are not up there fighting for a presumption of innocence but fighting for conviction.
Sally, the prosecution is supposed to be on a search for the truth. They are beholden to uncover evidence that proves that they have discovered facts to prove guilt, not fight for a conviction of whomever is accused. This is an ethical obligation and is cited in the Prosecutorial Code of Conduct.
Here is an short and interesting article, authored by a prosecutor, that presents a view different from the one you have outline
All prosecutors are taught that the paramount ethical standard of a prosecutor is "to do justice, not simply to convict." ... With prosecutorial discretion comes the responsibility to use that discretion wisely and fairly. Therefore, the printed ethical rules should be considered just a starting point. Prosecutors should be circumspect about their duty to seek justice, and should continue to fill in those "gray areas" of prosecutor discretion with their own ethical standards.
Prosecutorial Ethics Mark S. Pollock* It has been stated that the law is whatever is forcefully asserted and plausibly maintained. In our adversarial system, the role of the lawyer is to zealously and forcefully assert and advocate the position of his client, and to win. Prosecutors are lawyers, hired to represent the People of the State and to act as the people's advocate. But prosecutors fulfill a second, equally important function. An honest prosecutor is the strongest fortress and protection for the integrity of the Constitution of the United States. The role of a prosecutor, then, serves two masters. First, the prosecutor has an obligation to attempt to present evidence to courts advocating the people's position, upholding the law, and obtaining convictions for violations of the law. Second, and no less importantly, the prosecutor has an obligation to oversee the investigation of crime. Where it appears that the investigation has violated the constitutional rights of suspects, the prosecutor then has an obligation to the People of the State to take whatever action is necessary and appropriate to remedy those violations of individual rights. This is called prosecutorial discretion, the right and sometimes the obligation of a prosecutor to reject a case which has been referred by a police agency because rights have been violated as part of the investigative process. There are those prosecutors, who, like defense attorneys, feel that their ethical obligation to the client (the people) is best served by doing whatever is necessary to obtain a conviction. They therefore do the best they can to obtain the maximum penalty for the violation. However, there is another school of thought. There are those prosecutors who feel they serve a higher master. There are those prosecutors who take very seriously the oath to defend the Constitution of the United States and the Constitution of the state in which they were sworn. These prosecutors perceive their duty and obligation as one of presenting honest evidence to a court in order to obtain justice. They perceive that their role often may be served by not filing a case or by dismissing a case when evidence of impropriety arises in the course of the development of the case.
This second perspective is not to be confused with the concept of "coddling criminals." The wise prosecutor seeks to serve all three functions by obtaining a conviction and an appropriate penalty, thereby benefiting the society which pays him. Again, in requesting a sentence and a commitment of a defendant after conviction and bowing to public outcry instead of taking into consideration the need for rehabilitation of an offender, the prosecutor may be overlooking his ultimate obligation. The prosecutor must not act as a private attorney on behalf of victims of crime, but rather, as an objective advocate on behalf of the interests of the society itself and all the people. No case, no facts, no sentence, no victory in the name of the people, can be so compelling as to warrant the discarding of our constitutional integrity in order to obtain it.