This reader offers a complaint:

Jonah, I’m all in favor of ignoring international law
in American jurisprudence (except for treaties and
executive agreements, which aren’t really
international law at law, insofar as they’ve become
domestic law upon ratification/order).
But it is important to note – and I doubt I’m the
first to remind you of this today – that some
countries’ laws, while foreign, are still fairly
relevant to American jurisprudence.
That’s because America is a Common Law country, just
like (in most respects) Britain, Australia, New
Zealand, and Canada; and (in some respects) Jamaica
and Zimbabwe, among others.
Certainly, an early 21st century Zimbabwe case does
not have the jurisprudential value of a mid-18th
century British case. But, insofar as the Zim case
might be fruit on a branch of the same tree Justice
Kennedy swings from, it might be – sort-of – relevant.
It wouldn’t be controlling or, probably, very
persuasive; but I do think it is essential that your
readers understand that an American court’s reference
to a case from a country that used to be a British
colony is (probably but not necessarily!) better than
referring to a treaty that we haven’t ratified or to
which we’ve attached reservations.
Thanks for your time.

To a certain extent, I agree that common law questions are worth reading. And I really wish I could remember Scalia’s response to this point when it was raised at that AU event with Breyer.

However, the spirit with which these Justices have engaged foreign law — at least from what I’ve read — is different than the above criticism would suggest. Justice Kennedy was not reaching back to ask what Blackstone had to say about this or that. He was looking at international elite opinion and citing all sorts of other soft sources from outside the law. Indeed, public opinion in other countries has — I believe — been raised as a relevant factor in the Supreme Court’s decision-making of late. That’s outrageous.

Maybe Shannen, Ramesh, Jon or Andrew McCarthy have more to add.