I find this a little puzzling. Prosecutors are the source of impunity for police criminality. Prosecutors depend on police to make their cases for them, so they are not only ineffective at controlling police misbehavior, they generate impunity through their reluctance to harm their relationship with the self-protective paramilitary service on whose cooperation they depend. No ethical prosecutor wants to make knowing use of perjured testimony. No sentient prosecutor is unaware that some police lie under oath frequently, and that very few police are unwilling to lie under oath at all. So sentient prosecutors have an ethical interest in remaining as ignorant about police misbehavior and untruthfulness as possible. In general, police are the only criminals prosecutors systematically decline to prosecute. If you have a strong understanding of the patterns of wrongdoing by police, why would you want to have the only job in the system in which you are going to be practically compelled to be complicit in that misbehavior?