Why? Two reasons:
1) In 1964, it was a U.S. Supreme Court ruling that held in favor of the Cuban government in a case concerning that government's compensation-less expropriation of American property. The case turned on the application of the "state action doctrine", which holds, essentially, that the acts of foreign government's inside their own territory will be considered valid and legitimate.
2) In addressing the question of whether expropriation of property violates customary international law, the opinion notes and gives authority to the opinions of both communist and newly independent countries which argued that such a position was unfair and in service of "imperialist" interests:
There are few if any issues in international law today on which opinion seems to be so divided as the limitations on a state's power to expropriate the property of aliens. There is, of course, authority, in international judicial and arbitral decisions, in the expressions of national governments, and among commentators for the view that a taking is improper under international law if it is not for a public purpose, is discriminatory, or is without provision for prompt, adequate, and effective compensation. However, Communist countries, although they have in fact provided a degree of compensation after diplomatic efforts, commonly recognize no obligation on the part of the taking country. Certain representatives of the newly independent and underdeveloped countries have questioned whether rules of state responsibility toward aliens can bind nations that have not consented to them and it is argued that the traditionally articulated standards governing expropriation of property reflect "imperialist" interests and are inappropriate to the circumstances of emergent states.
The disagreement as to relevant international law standards reflects an even more basic divergence between the national interests of capital importing and capital exporting nations and between the social ideologies of those countries that favor state control of a considerable portion of the means of production and those that adhere to a free enterprise system. It is difficult to imagine the courts of this country embarking on adjudication in an area which touches more sensitively the practical and ideological goals of the various members of the community of nations. (428-30)
I'm not expressing an opinion as to either existence or normative desirability of a customary international legal norm against expropriation without compensation. Rather, I thought it noteworthy both that Cuba won a case like this in American courts, and more broadly, that an American court openly considered the implications of certain perspectives normally considered quite radical and out of bounds in our public discourse.
Generally, this is how one is supposed to approach questions of customary international law, and of course, this is the double-edged sword of that institution: since it represents the customs of the entire world, and there is much disagreement in the world over a variety of customs which we often take to be touchstones of modern human rights standards, generally international law will always be tied to the behavior of its most regressive members. Canvassing the international community in order to determine international custom doesn't mean just looking at countries generally in line with American interests or values, and I thought this case illustrated that in uncommonly vivid fashion.