Some of the logic is that bad police behavior is widespread enough that every "good" cop works closely enough with at least one "bad" cop, and yet via observation of outcomes, does not correct that bad cop's behavior (or doesn't correct it enough). They're thus complicit AKA bad cops themselves, and thus there are _no_ good police - everyone is sufficiently tainted.
The Godwin's Law version of this logic at work elsewhere is the German saying "if there’s a Nazi at the table and 10 other people sitting there talking to him, you got a table with 11 Nazis."
The folk wisdom encoding "a few bad apples spoil the bunch". Doesn't matter if they were good apples going in, they're all bad apples now: the bunch is spoiled.
(Not expressing an opinion on the correctness of this thinking, just explaining what I know of it)