When discussing a novel issue of law, courts can look at arguments from pretty much anywhere. These arguments are not controlling legal authority, but are what is known as persuasive legal authority.
I actually don't see a problem with this passage. This is just a wide-ranging discussion of international trends and SCOTUS's history of ruling on the 8th Amendment.
Yes, we can all look forward to the time when the Justices recite from the Koran.
I tend to agree. The majority isn't claiming they are bound by international law or trends, but rather, is generally discussing the laws and trends in other countries as they relate to criminal justice under the 8th Amendment. That's not much different than looking to the Magna Carta, Common Law of England, or even the Bible for perspective. Now if the SCOTUS claims it is bound to construe the Constitution based upon international laws and trends, then I have a real problem.
"...and in the course of discharging that awesome responsibility purports to take guidance from the views of foreign courts and legislatures. Because I do not believe that the meaning of our Eighth Amendment, any more than the meaning of other provisions of our Constitution, should be determined by the subjective views of five Members of this Court and like-minded foreigners, I dissent."