The punishment comes after the investigation, not before.
What exactly do you think this investigation is going to yield? "It looked like he may have had a weapon" or "He made a threatening comment"? Neither are reasons why the officer should have shot the man.
Edit: and yeah I agree cops don't get convicted often enough. Either way a real investigation is going to take time.
Also, these separate serial investigations seem like a powerful incentive to brush over the criminal inquiry as quick as possible - figuring wrongdoing can still be punished by the second investigation, and also not wanting to delay it.
Though being put on house arrest and being treated as on duty whilst on leave is on dodgy grounds I am surprised the police unions haven't challenged this in court.