I'm not sure we are REQUIRED to do so, either. It seems to be a regional thing, or dependent on the location.
Household water heaters .... in the cities you seem to find them hooked up to a real disconnect on the wall using flex. Out in the suburbs,you're likely to just find a tail of cable poking out a hole in the wall.
Personally, it is my thought that good design would allow for the water heater to be replaced (or repaired) without requiring the plumber to take apart the electrical connections. I also like having a way to disconnect the heater right where the plumber will be working.
