The EPA established the federal S15 (ULSD) regulations for the United States. Individual states may adopt more stringent requirements than the federal mandate.
This is the case in California. Specifically, the state is mandating a tighter timeline for the introduction of S15 at the terminal and retail level. They have also chosen to require S15 to be used in all vehicles (e.g. both on-highway and off-road) as opposed to EPA's initial requirement that S15 must be used in only on-highway vehicles.