U.S. could still be bound by UNCLOS law even if it hasn't ratified the treaty
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Public international law governs the relationships between states and, on occasion, the relationships of states to international organizations. Nations commonly assume obligations to each other through treaties; however, a state may be bound by a norm of customary international law notwithstanding its failure to enter a treaty. Customary international law is created in a variety of ways, including by treaty provisions adopted and followed by sufficiently large numbers of states as a matter of legal obligation. Customary international legal obligations also give rise to an array of international remedies. Thus, the fact that the United States has not ratified UNCLOS does not necessarily mean the United States is free—as a matter of international law—to ignore particular UNCLOS legal norms or processes. If, in fact, the United States is under an obligation to comply with an UNCLOS provision that has also become customary international law, failure to comply could give rise to international liability and subject the United States to international legal remedies.
There is a strong case to be made that the United States is obligated under international law to comply with UNCLOS’ seafloor regime despite the fact that the United States has never ratified the convention. The most fundamental and compelling reason the United States is bound by UNCLOS’ regime for the extended continental shelf is because, quite simply, the United States says it is bound.170 Moreover, even though the United States has not ratified the convention, as a signatory to the revised deep seabed mining provisions, the United States has incurred an international legal obligation to not act contrary to the “object and purpose” of the treaty.171 In light of the prominent role given the deep seabed mining regime in the convention and its necessary and practically inseparable relationship to the extended continental shelf regime, the United States is arguably not permitted to act in any way that would undermine these central provisions.
States, corporate entities, and NGOs all have incentives to challenge unilateral claims by countries to resources outside the UNCLOS regime. Knowing this, U.S. corporations are reluctant to risk the liability involved in pursuing these claims, to the detriment of the U.S. economy.