The most common interpretation I've heard for this phrase is a contrast to the "few bad apples" meme, because police consistently show solidarity in defense of those "bad apples." If you're getting beaten by a cop, and their partner is just standing by and watching it happen, they're both bastards.

That said... the cops who made the news for shoving other cops off from kneeling on protesters' necks? They were good cops in that moment. Even if they were doing it to protect their precinct from scandal.