Prosecutor is Selective in Concern for Privacy Rights

The linked article figuratively twists a blade.  I was recently involved in a civil case in which the police investigator avoided any evidence that might call the complainants credibility into question, or that might allow the accused to prove her innocence.  Based on the article linked to below, that type of thinking and bias in not limited to the particular investigator, not to police, as opposed to prosecutors.  If a technique or methodology can be used both to convict and to exonerate, on what basis may it be argued that is serves justice to inculpate, but not to exculpate?

The Pros and Cons of Lies in Politics

The author of this article makes the case that lies have a legitimate place in politics, but lying more often is merely an expedient tool for coping with an ignorant and/or intellectually lazy public that, too often, prefers reassuring lies to the truth.