You would have to ask the code officials about that. But I suspect it has to do with the degree of the hazard of what can backflow from a lawn sprinkler system. The water that pools around the sprinkler heads can have insects, pesticides, herbicides, fungicides, fertilizer, worms, and other contaminants. Which would make it a high hazard backflow issue which according to the EPA codes would require an air gap (which in this case wont work) or a RPZ valve installed.
Originally Posted by johnjh2o1
There has been many cases where people have gotten ill from drinking water that has gotten contaminated by the lawn sprinkler systems. I was told about one where a local high school football team gotten sick from drinking water that was contaminated with animal waste.