the gov't dude said that there is the "chance" that if a hydrant blew, or a firetruck hooks up, that there would be a negative pressure. Once the water leaves the house meter, they don't want it back into the system. I can appreciate their concern, but he's talking IF my auto shut off fails, IF my tank fills over the fill pipe, and IF that happens at the same time whatever causes the freak neg-press, THEN it could siphon unpotable water from my rinse tank. IF all that happened.
Its a hell of a lot of time and money to cover an IF that I've never seen happen in all the 15 yrs I've been cleaning. Have to remember, they want it tested and re-approved EVERY year, that means for some lucky plumber, he's got gov't license to take our money every year.
Eventually every house/busn is supposed to get one of these valves at the meter. Some lucky plumber will be busy ALL year JUST inspecting and testing valves. Gov't job security, paid for by the public.