In the Shadow of the Great Charter

In the Supreme Court's 2008 ruling on whether Guantanamo detainees could be barred from U.S. courts, Justice Anthony Kennedy cited the U.S. Constitution, of course. But he also linked the decision to the Magna Carta. Why would a twenty-first century judge, even under the extraordinary circumstances of the "war on terror," invoke a document signed by an English king in the thirteenth century?