Those are 'real' accidents; and if some are not, like DUI, there are laws against them that are enforced. If tomorrow, society stops charging DUI offenders people would protest too.
That's not the case with police brutality. We have seen again and again the police face no consequences for their brutality. That's what the people are protesting against. If every one of these cops were appropriately fired, charged and jailed, this wouldn't be a big of an issue. That simply does not happen.
The second point being, theoretically people trust cops to keep law and order and to ensure safety. So it is rightly expected for them to have higher standard of conduct and the fact that this does not happen is a systemic failure.