USA Today gives a balanced analysis of the controversy.
I think Scalia has the stronger argument. It’s appropriate and valuable for the legislative and executive branches to pay attention to other countries. But, the SC is supposed to be interpreting the US Constitution. Using foreign courts gives them yet another excuse to enact their policy preferences through judicial fiat.
Note Scalia’s parenthetical phrase about ignoring countries that the Justices do not agree with. If the Supreme Court were totally consistent, they could rule that torture is NOT “cruel and unusual punishment” because it’s still used in many nations. Of course, that’s purely hypothetical.
It’s noteworthy that Justice Kennedy didn’t need to cite the European Court of Human Rights in Lawrence vs. Texas. There were adequate grounds for his decision in the US Constitution. He went out of his way to emphasize that he was defining basic human rights, not basic US Constitutional rights. In other words, his view seems to be that the US Consitution guarantees whatever human rights this year’s Supreme Court thinks are important.