That might be justified in the case of a law enacted by a state not not in the case of a law that is enacted against the entirety of the nation. In those cases the original intent of SCOTUS and its primary purpose comes into play. And that purpose is to determine the constitutionality of the law. It should be noted that SCOTUS does have the right not to hear a case, but by taking the case up they are then obligated to render a desicion based on the briefs brought before them. In the case of Obamacare the tax justification was never part of the federal govt. reasoning for the constitutionality of the law, and as such Robert overstepped his limits in considering it for his decision.