This whole uproar over citing international legal authority is mind-bogglingly stupid. It amounts to insisting that courts aren't allowed to be persuaded by international sources. That's utter bullshit. Persuasion depends on the validity and soundness of the reasoning and there's no reason that U.S. courts can't look to foreign jurisdictions to see what works any more than sovereign state courts can't look to other state courts to see what rules they have adopted. No one is suggesting that foreign law is binding on U.S. courts and the people who are defaming Koh probably know that.
Koh’s unremarkable position is that rulings by non-American courts should be considered, when applicable, as persuasive authority in deciding an issue before an American court. Unless required by treaty, American courts would have no obligation to follow the decisions of any foreign courts. They should merely consider how they addressed the same issue. Not only is this not controversial, it is also common sense.
For example, if Philadelphia’s City Council is considering the effect a smoking ban will have on local businesses, it would be foolish not to look at how similar bans affected business in New York or London. Similarly, if a court is considering whether mentally retarded defendants should be sentenced to death, it might want to see how other democracies with similar values have addressed the issue.
This is little different than a Pennsylvania court citing the opinion of a New Jersey court in support of its decision. To do otherwise would be to ignore the world around us and deprive courts of a valuable tool in understanding the consequences of their decisions. Unless you believe that judges should pay no attention to the results of their actions, Koh’s position should not alarm you.