Convention, nor ordinary criminals. But you and DiffEll are incorrect that this category is something new and illicit. The category is "spies and saboteurs", who are presumed to work in the service of some nation or organized ideological cause, as opposed to their own pocketbooks; they employ "criminal" or extra-military methods nonetheless. Most countries have long treated such customers as subject to summary execution, or indefinite imprisonment without hearing. The nations and organizations which send them out in the first place are well aware of this, as they themselves are.

Shoot, even Jim "Mr. Clean" Phelps himself always had to hear that ominous caveat at the end of his tape-recorded mission instructions---"Of course, should you or any of your I.M. Force be caught or killed, the Secretary will disavow any knowledge of your actions". A KGB motto was "Death to Spies". The Mossad didn't piss around asking for legislation, treaty, or official Israeli or international sanction to despatch the Black Sptembrists, any more that those pigs themselves did before Munich.