The milk fight, which is being watched in the industry from coast to coast, started because Mr. Hettinga runs a rare hybrid operation. Most dairy businesses either only produce milk, or only process it. He does both. As a result, he falls into a protected class that isn’t bound by an arcane system of Depression-era federal rules. Under it, milk processors selling into specific geographical areas, which cover most of the country, must all pay into that area’s pool for subsidizing milk prices. But so-called producer-distributors have always been exempt.