"prevent investigators from pursuing other cases of misconduct revealed during an investigation"
Who does that protect? That's like arresting someone for stealing a candy bar but not being able to do anything if you found out they stole the whole store.
It's so you don't hear stories like "He was unarmed and did nothing wrong but was shot and killed by police." being followed up with "During investigation it was found he had murdered 2 children years ago, held his grandmother hostage, spent 8 years in prison for drug/gang charges, and has warrants for his arrest in 15 states."
I can understand why they would argue for something like this - as it's essentially poisoning the well.
If it was found the person hadn't done any of that previous stuff, would it make them being shot while unarmed "not okay" but because they did all that stuff previously it is suddenly "okay"?
It's distracting from the issue of "this man was shot and killed by police" by trying to justify it with the man's past offenses - even if the past offenses are not relevant to the man being shot.
Note: Unarmed does not mean not dangerous, but that's an argument for another time.
E:
In response to below, I somehow missed that context when looking at it, so apologize for my reply missing the bigger context.
This policy is in regard to the police officer. If the officer shoots someone and is investigated, evidence of prior crimes the police officer committed can not be prosecuted.
It certainly doesn't apply to normal citizens. That's why the police sometimes try to catch someone on a minor violation to obtain a search warrant for their home in hopes of uncovering larger crimes.
"prevent investigators from pursuing other cases of misconduct revealed during an investigation"
Who does that protect? That's like arresting someone for stealing a candy bar but not being able to do anything if you found out they stole the whole store.