Death came without warning to Halatau Naitoko, on a flat stretch of motorway near Western Springs. A police chase left an innocent teenage father dead and a city stunned. Naitoko, 17, was killed by a shot fired by a police marksman.

This week, the police announced that no one would be charged over the young man's death. The decision followed "a thorough homicide investigation", they said, a choice of words intended to signal that it was not an internal inquiry. Police were keen to stress that Naitoko's death was treated like any other homicide.

But it wasn't. With other homicides, the police establish who is responsible and lay charges. The courts decide whether or not the homicide was culpable - whether the person responsible is guilty of manslaughter or murder.

This is not a quibble, but a fundamental plank in our constitutional arrangements: Parliament makes the law; the police enforce it and apprehend those who break it; the courts decide whether the police have proved their case. The police have discretion to decide not to act - if an offence is trivial, for example - and they make such decisions every day. But when somebody dies, such police discretion becomes a luxury we dare not afford.

Police have been subjected to a good deal of opprobrium in the past for playing by the book. They charged Northland farmer Paul McIntyre with injuring with reckless disregard when he fired on intruders trying to steal his farm bike. It took two trials and 2 years before McIntyre was cleared. Otara liquor store owner Virender Singh was charged with assault when he took a hockey stick to two teenagers who were trying to rob his store. The case was thrown out at depositions. Singh and McIntyre were understandably angry, but a fundamental principle was at stake: that police are not judges or juries.

Yet they seem to lose track of that idea when it is one of their own under investigation. They declined, after a "careful and through investigation" not to charge the officer who shot Stephen Bellingham in Christchurch in September 2007. Likewise Keith Abbott, who shot a berserk Steven Wallace in 2000.

History has proven that, where cases have no merit, they will be thrown out at an early hearing. Meanwhile, serious questions are answered and, more important, a fundamental principle is upheld: justice is not done unless it is seen to be done. The uncomfortable sense persists that the police are investigating themselves and finding nothing wrong.

Police will always have available to them defences on which the average citizen may not rely. Required by the terms of their warrant to exercise lethal force when needed, they are entitled to a degree of latitude when things go wrong. Public opinion gives them that. But public opinion does not decide whether the law has been broken: the courts do.

Community sympathy for the police has been high since Len Snee became the third officer in 10 months to die in the line of duty. But that does not mean police use of lethal force should escape scrutiny. The coroner and the Independent Police Conduct Authority both have investigations ongoing into the motorway tragedy. It is to be hoped that they will give a degree of transparency into those horrific events. The "thorough homicide investigation" has, for the public, raised many more questions than it has answered.