Here are some excerpts from an old law review debate about the constitutionality of CUSFTA Chapter 19:
Alan Morrison of Public Citizen:
If the meaning and applicability of the laws of the United States dealing with antidumping and countervailing duties are enunciated and given legally binding effect in decisions directly affecting the rights of private parties, as well as the government, the Appointments Clause requires that such decisions be made by officers of the United States for whom the President and the Senate are accountable. Moreover, it surely forbids final administrative decisions that are made by officers of the United States, appointed by the President with the advice and consent of the Senate, especially when the purpose behind the creation of the panels was to alter the result under the very laws that the members of the panel are supposed to be upholding. The use of these bilateral panels is dangerous to our system of democracy, even in this limited context, but their potential application in other international agreements requires that the courts promptly step in and hold the panel review scheme unconstitutional before it spreads to many other areas of law.
Jim Chen, currently a law prof at Michigan State:
As salient exceptions to free trade between Canada and the United States, antidumping and countervailing duty laws can undermine the FTA. This concern, no doubt, led both nations to substitute binational arbitration for traditional judicial review. Yet a government based on checks and balances "rank[s] other values higher than efficiency." Both Article III and the Appointments Clause pose substantial constitutional obstacles to the FTA. These obstacles protect the citizens affected by the trade laws by preserving institutional safeguards built into the Constitution. Like all other citizens in the United States-Canadian free trade zone, these parties will reap enormous benefits from the FTA, but they should not be forced to sacrifice rights assured them by the separation of powers. Adherence to the Constitution's structural requirements ultimately benefits all by ensuring that the compromises emerging from the political branches retain their crucial core of accountability. As Justice Scalia has noted, "in the long run the improvisation of a constitutional structure on the basis of currently perceived utility will be disastrous."
Bill Davey of ... well, you all know Bill Davey:
... I wish to make essentially three points: First, the dispute settlement provisions of Chapter 19 of the United States-Canada Free Trade Agreement (FTA) do not violate even a strict reading of the Appointments Clause. Second, if a court deemed the constitutionality of the Chapter 19 procedures to be a close question under the Appointments Clause, there is ample precedent for finding them to be constitutional because of their connection with foreign affairs, where the Supreme Court has traditionally been less concerned with separation of powers issues. Finally, upholding these provisions is consistent with and reinforces representative democracy.
Beyond the constitutionality issue, this from Davey was also interesting:
Under Chapter 19, the decisions issued by panels are in reality only advisory, which suggests that the panels are not exercising power pursuant to U.S. law. While it is true that the United States has agreed to abide by the rulings, it cannot be compelled to. To put it another way, a prevailing party in a panel decision could not get an enforceable order from the panel or a U.S. court that would direct the Commerce Department or the International Trade Decision to implement the panel ruling. Obviously, a decision by those agencies to ignore the U.S. commitment might lead to a denunciation of the FTA by Canada, but that does not change the fundamental power relationships. The panels, and those favored by their decisions, in the end cannot make the United States do anything. As such, it cannot be said that the panels exercise significant authority pursuant to U.S. law.
I'm a little surprised that there has never been an issue with U.S. agencies refusing to follow NAFTA Chapter 19 rulings (at least as far as I've heard). In addition to the political reasons Davey suggests, is it also because the agencies can often find a way to comply with the rulings that allows them to achieve a similar result with new reasoning?