manta49, an arrest is not required for an investigation to take place in my neck of the woods.
Are you claiming that in your country, the police MUST make an arrest in order to initiate an investigation? That seems entirely backwards.
OTOH, if they do NOT have to make an arrest in order to investigate, then why are you defending the arrest of the couple in this case? The Crown found there was no cause to charge them; so, why should an arrest be automatically assumed?
Note: I am not claiming that people involved in good shootings are never arrested over here; I am claiming that in cases where prima facie evidence indicates that an act was justifiable, the police are generally able to use their discretion. It does not seem such discretion was used, in your country, in the incident under discussion here.