It is even worse.
The prosecutor's job is not only to prosecute crimes,
the prosecutor has an AFFIRMATIVE DUTY to dismiss cases in which the evidece does not support the accusation.
Contrary to TV fiction they are not supposed to win at all costs. (or as in Nifong's cases use them as disposable pawns because they have the wrong skin color)
They represent the people. Their job is every bit as much to leave those citizens alone for whom there is insufficient evidence to assure a clear conviction as it is to secure conviction of the clearly guilty. A DA looking at someone cross-eyed can set him back $100,000 in legal fees. There is much more at stake than just a "fair" trial. The trial is unfair if it never should have happened in the first place.