There are some interesting notions, misconceptions and accusations of conspiracies floating around in this thread. I suppose that all of the discussion of common law by sovereign citizen types has tainted the topic a bit. But whatever perceptions people have of common law today, thanks to the gold fringed flag crowd, aren’t really relevant to the historical origins of this legal peculiarity.
If I recall correctly, the Coroner goes back to medieval England. Sheriffs were elected / anointed locally and were often corrupt or simply members of the good old boys club. In order to exercise more centralized control and reduce corruption the king imposed a system of appointed coroners to act as his johnny on the spot. My English history professor described the origin of the word as “crowner” implying their purpose; he also described the coroner as the only person empowered to arrest the sheriff.
After the revolution we inherited common law through reception statutes and precedent. Of course much of it has been modified or swept up into code law today and I am unsure of its exact status. My perusing of Pollock and Maitland on the History of English Law for class didn’t cover very many recent occurrences.
Ask a lawyer, not a history major, if you want to know about it today.
TLDR: Relic from English medieval law, modern relevance varies by jurisdiction, feds arrest anyone they want etc.