Trumpy, nail on the head- it's the ground water! Human sewage contains specific-to-human bacteria and viruses. Analysis can tell the difference between, for instance, beef, pigs, chickens, kangaroos [hat tip to Reno!] & people. Leaks into the surface soil spread as much as 30 yards and can be detected by identifiable pathogenic bacteria counts. That's why a well should be at least 40 yards from any sewerage pipe. If a sewage leak enters the underground aquifers in your area, [ and these can be very close to the surface in some places], then the spread of bugs could be miles!
Having said that, it's surely overkill to make sweeping laws banning all HO plumbing or electrical work. I suspect that in many cases it's local gummint flexing their tax gathering muscles rather than any altruistic 'safety' motives.
As a local Maire, [the French are more laid back than most in these things], said to me once-
"You built a house on your own land. It fell over. What has that got to do with me!?"
