Just amazes me how the electrons know the difference and can decide which conductor to use depending on who owns the equipment.
Actually this sort of unwritten rule started before the changes in 2008 that required us to use an EGC with feeders.
I had a situation where I was connecting a couple mobile home services, POCO had a meter/main that they supplied on a pole that was right next to the home. Inspector would not let me use that disconnect as the required service disconnect because of the fact that POCO's providing such equipment can change that equipment out for various reasons. The typical service from rural POCO's in this region usually leaves you with a disconnect from their source but is not always something with overcurrent protection. For that reason they wanted me to provide an additional 200 amp service disconnecting means between the POCO disconnect and the mobile home.
Rural POCO's are also known to commonly provide an unlisted unit containing a meter and double throw switch (transfer switch) if customer wants to pay extra for it - this is most common thing you currently do see that contains no overcurrent protection that often is replacing a meter/main. It cost extra but they can still install it for less then I can install a NEC compliant manual transfer switch, and is in a nice looking single cabinet instead of multiple items cobbled onto the pole. I have one on my service pole myself - at the time it cost me nothing because of electric heating incentives out there at that time. I think today it would cost me something - but still less then I could buy transfer equipment for.