A look at the legislative history does not support the notion of a subjective, political exemption for politically-motivated pirates.
Kevin Jon Heller’s argument that political motives are excluded from piracy rests less on the Law of the Sea Treaty itself than on its intellectual predecessors, the League of Nations Report of 1927 and the Harvard Draft Convention of 1932. The ILC Commentary to the Law of Sea Treaty specifically endorses the latter, saying that “in general” it agrees with the Draft Convention (it does not provide such deference to the former.
The Harvard Draft supports the lack of any subjective, motive-based inquiry. Here is exactly what it says in its commentary on the “private ends” part of the definition of piracy:
[A]lthough it is true that the typical pirate of fiction and tradition was an indiscriminative plunderer, expediency and not traditional epithets or the fancy of traditional concepts should direct the definition of the common jurisdiction over piracy, and every consideration of certainty in prosecution and of assured protection in places outside the territory of all states argues that the jurisdiction to seize and to punish a robber or a killer for private ends should not depend on whether the offender had by acts or words displayed an intent to plunder or slay only once or oftener, or on whether he intended to attack only the
citizens of certain states and their ships and other property, or to prey on the people and commerce of all nations indiscriminately. Such matters of collateral intent of an offender (often uncertain and indistinct) and of his transactions other than those involved in the case at hand, are very unsatisfactory as elements in a basis of state jurisdiction.
On the other hand, the language in the Draft that Prof. Heller says best [...]