Here is what I think is the key quote from Kennedy's dissent in the California Prop 8 case

From Justice Kennedy's dissent:

The Court’s reasoning does not take into account the fundamental principles or the practical dynamics of the initiative system in California,which uses this mechanism to control and to bypass public officials—the same officials who would not defend theinitiative, an injury the Court now leaves unremedied. The Court’s decision also has implications for the 26 other States that use an initiative or popular referendum system and which, like California, may choose to have initiative proponents stand in for the State when public officials decline to defend an initiative in litigation. . . .

So the US Supreme Court claims that it knows more about how the California laws should be interpreted than a unanimous decision by the California Supreme Court.  But worse, as Justice Kennedy says, if the politicians in office don't like a decision, they can just encourage a lawsuit against the law and then refuse to defend it in court.