Nah it's because there are people who actually believe that shit so it's hard to gauge sarcasm.
It's a lot like how people make jokes about landlords. A lot of the people are being sarcastic but there's enough of them that are serious that you can't really tell the difference.
The original intent of that metaphor is correct in your use, but it’s rarely recognized. It began as “a rotten apple quickly infects its neighbor.” Over time, it became “one bad apple ruins the bunch.” Now it’s used as just “one bad apple” to infer minimal or selective corruption, completely discrediting the point of the analogy.
It's also worth noting the implication of the full phrase. If you remove the bad apples quickly enough, then you can save the rest. If you can remove the corrupt elements, then you can protect the group overall. If you leave them to fester then you'll have a lot more cutting required to clean up.
I only know like 2 dozen cops. One's an absolute blowhard, but the rest are decent people. I don't expect them all to be nice, because I've seen reports on TV otherwise; it'd be foolish.
The cherry-picking starts with US newsmedia. I'm glad our cops are different, at least.
The problem that brings up the idea of no good cops is, if these 23 other cops are good, and this one is bad, why is there still the bad cop? Why do the other 23 not push that bad cop out?