The American police force originally started as a formalized slave patrol to capture runaway slaves [0]. It's well-documented [1]. We can try to argue whether modern policing carries that tradition, but case [2] after documented case [3] keeps bearing out more of the same. It's been the topic of research [4] and pop culture [5].

[0] https://www.nas.org/academic-questions/36/3/did-american-pol...

[1] https://time.com/4779112/police-history-origins/

[2] https://www.britannica.com/biography/Rodney-King

[3] https://en.wikipedia.org/wiki/George_Floyd

[4] https://pmc.ncbi.nlm.nih.gov/articles/PMC7331505/

[5] https://genius.com/123154

You seem to be under the mistaken impression that the claim here is "majority of cops are white supremacists". Thats not the claim. The claim is that it is sensible to assume a cop is.

A very different bar. A minority of cops can be white supremacists and because of the power they wield it's still sensible to treat them like every interaction is with a a white supremacist. As an example, a cop can legally kill you in many cases (or deny you freedom or seize your assets). If you had, say, a 20% chance of encountering a cop who was a white supremacist it would be sensible to treat every interaction as if that were the case.

Consider how unevenly weighted the outcomes depending on whether you assume a cop is racist when factoring how sensible it is to assume they are.

Do you personally know any police officers? I do and, as a group, I've found them to be more racist than the general population. I don't know what the working definition of "white supremacists" is in this context but it doesn't make me blink.

We can't on one side ask for people to not make judgment based on statistics and on the other side saying that making a shortcut based statistics is valid.

Won't someone think of those who falsely accuse someone of kidnapping when they get a similarly ridiculous accusation against them?

This is part of why we have juries. The letter of the law must be nullified sometimes in the interest of justice.