@oralloy,
It may be that the case is so high profile, and there was such a history of complaints of excessive force against Chauvin that the Federal charges will be pursued:
- as a deterrance to other officers from racking up multiple complaints (if you get found guilty of a serious charge, we'll look at and prosecute any other similar complaints)
- as a deterrance to police departments (you're not actually doing your employees any favours by not addressing the cause of so many complaints);
- because there is a public interest in doing so (1. Maintaining confidence in justice - do they really ignore Chauvin's history after Floyd? Many will think it got swept under the carpet if it is not prosecuted. 2. the above two reasons)
Judges do sometimes hand down harsher sentences for their deterrance value. The charges may be for a similar reason - not strictly necessary, but able to be done, and so done for deterrance.
Edit: what would likely change things for the better is to introduce laws where:
- if a complaint was dismissed without substance (by the complaint investigators); and
- later a criminal charge for the same complaint is successful
- a court or independent body can determine if the investigator to be guilty of misconduct
- and apply sanctions or fines accordingly
Findings outside of criminal courts are held to the balance of probability standard, which is a lot lower than the criminal - so if a crime gets found to the criminal standard, where a complaint investigator allegedly could not find one to the civil standard, then there is usually a real problem.