I take it as clear that as a general matter we have the same right to speak freely about foreign affairs as we do about domestic matters. The fact that the President, sometimes referred to as the "sole organ" of the United States in foreign affairs, wishes to speak in one way about foreign matters does not deprive other Americans of the right to speak in different ways. Nor would Congress' concurrence with the President have this effect.
But as a general proposition the constitutional guarantee of free speech is not, in fact, understood as an absolute guarantee, but rather as a guarantee that speech will not be restricted except for a very good reason. The standard for what amounts to a very good reason can be very stringent -- say, "necessary (or narrowly tailored) to a compelling state interest" -- but it can be satisfied.
If we use the formula of "necessary (or narrowly tailored) to a compelling state interest," then the issues are "what's a compelling state interest" and "what's necessary or narrowly tailored to achieving that compelling state interest?" The decisions and judgments of the political branches of government might be relevant to both of these questions.
First, few doubt that defeating international terrorist threats to the United States is, today, a compelling state interest. But it is not self-evident that defeating every group designated as a terrorist organization is a compelling state interest. Not every group that has been called "terrorist" is automatically our enemy. At least one, the African National Congress, now is the governing party in a nation, South Africa, with which we maintain friendly relations -- the very happy result of the ANC's successful, generations-long challenge to apartheid.
Why is it in our interest to cut off support for a Kurdish organization or a Tamil organization that we have found engaged in terrorism? Both the majority and the dissent in Humanitarian Law Project no doubt agree that ending material support for designated terrorist organizations is indeed a powerful interest (Justice Breyer affirms this point explicitly, at page 7 of his dissent). I think the ultimate reason is that courts must view this goal as in our interest because the political branches of the government say it is. We, through our representatives in government, make foreign policy, and the country has a compelling interest in accomplishing its foreign policy objectives. The fact that our foreign policy may be far from a perfect expression of democratic choice doesn't really much change this analysis; it's still the only foreign policy we've got. The courts, in short, must generally defer to the political branches' identification of what we seek as a nation in the perilous world.
In roughly the same way, our political branches' choices presumably affect the application of the prohibition on treason. Article III, section 3 of the Constitution says that "[t]reason against the United States shall consist only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort." Exactly when speech on behalf of a wartime enemy might constitute "Aid and Comfort" I am not sure, but I think it must be the case that it is up to the political branches of our government, at least in most situations, to decide who our "Enemies" are.
Once we've identified our foreign policy objectives, what is necessary or narrowly tailored to achieve them? The material support statute can be understood as embodying the political branches' view that cutting off all material support to designated terrorist organizations is in fact necessary to achieving those objectives. If that's what the political branches have concluded, and they're the branches responsible for and expert in foreign policy, should their judgment be deferred to? If so, then material support in the form of speech could be criminalized, because doing so is necessary (per the judgment of the political branches, to which the courts defer) to achieve the compelling governmental interests (as identified by the political branches). More on this line of argument in a future post.