Justice Kennedy tried to articulate
It is proper that we acknowledge the overwhelming weight of international opinion against the juvenile death penalty, resting in large part on the understanding that the instability and emotional imbalance of young people may often be a factor in the crime.
The opinion of the world community, while not controlling the outcome, does provide respected and significant confirmation for our own conclusions.
It does not lessen our fidelity to the Constitution or our pride in its origins to acknowledge that the express affirmations of certain fundamental rights by other nations and peoples simply underscores the centrality of those same rights within our own heritage of freedom.
Which raises two questions: Isn't weighing the "instability and emotional imbalance of young people" exactly the kind of thing that legislatures--the peoples' elected representatives--are supposed to do? And why, if the Court's conclusions are based on the Constitution and laws of the United States, is "the opinion of the world community" a factor in the Court's conclusion?
It does "lessen [the Court's] fidelity to the Constitution" when the Court gives the actions of foreign governments priority over the text of the Constitution, the laws enacted, in this case, by the legislatures of 20 states, and the clearly expressed preferences of the majority of Americans. With all due respect to the Court's majority, there is simply no coherent rationale for counting the "enlightened" opinion of foreign governments as a factor in Constitutional jurisprudence.
Read the whole thing.
New Sisyphus has a long essay on the case and the ruling but it is well worth reading in its entirety.