after St. Louis police unions instructed officers to publicly display The Punisher's insignia (the mark of a lawless, fascist murderer) the comic book community was quick to point out the stupidity, and the frankly horrifying message sent by supposed peace officers endorsing a maniac. So it's a good thing The Punisher personally confirms he hates cops who see him as a friend… making every cop who "wears his mark" or calls themselves a "fan" look like shameful fool in the process.
-- Andrew Dyce, The Punisher Confirms: He HATES Cops Who Support Him
The point is that when they label themselves with a symbol or name, they up to some criminal shit already or before long.
And yes, sections of US police are corrupt as cops get away with murder on a daily basis.
Corruption doesn't need to mean they are on the take (although there's probably plenty that are), it can also simply mean that they turn any level of blind eye for their own or external groups.