When it comes to criminal prosecution individual citizens are on arrears. Save for the most affluent among us citizens accused of a crime have not the resources the state has at its disposal. So if law enforcement officers are involved with tampering of evidence or even with outright perjury, it is very hard, if not impossible, for a citizen to prove his or her innocence.
Therefore we are in favour of mandatory prosecution of law enforcement officers against whom there are serious suspicions of perjury and/or tampering with evidence. Too often those are protected by the system, but no one is infallible. And the knowledge that even law enforcement have to answer for their action, would increase the public trust in them.