I also agree that the likelihood of getting black water back into the public water supply is very remote at best. BUT. If there is a way, someone will do it. And it is possible under very specific conditions.
When a public water system is shut down, water in the lines runs to the lowest point in the lines as people open water valves and the remaining water drains out. That pulls a vacuum on the upper parts of the water line. If you open a faucet soon after the public water supply is shut off you will probably either have water trickling out if you are at a low point, or hear it sucking air if you are at a higher point. If someone in the campground connected their flush hose, did not drain their tank first, their black tank water level was above the spray head level, they opened the water shutoff valve to the flush line, and public water happened to be shutoff at the time and pulling a vacuum on the line, black water "could" be sucked back into the flush line and into the public water system. That would be something most likely done by someone new, or by someone who knows no better, or who simply does not pay attention. But the possibility is still there. Highly unlikely, yes. Possible, yes. That is why outdoor faucets and sprinkler systems are required to have anti siphon valves.
The codes are not there for us who know better, they are there to protect us from the ones who don't.
