If he's so found of european justice, perhaps he should resign his seat and apply for one in europe. In case he didn't notice it, he's a member of the UNITED STATES Supreme Court, not the International Court.
The Court has the right to invoke other municipal law but ONLY when dealing with matters of international law. In any case, their authority is limited to case under the Constitution and laws of the United States. Can you imagine the outcry if Scalia or some Catholic jutice were to invoke Canon law?