There is no need to investigate very far to know that this kind of set up was a very current practice in the 80s, within the "international" operators, and if its ethic can be questioned, employees were knowledgeable, responsible of their commitments and in the vast majority, happy to get this job.
I can't believed that the charges of concealed work, illicit work, social dumping could apply here.
Of course this type of contracts gives an almost complete authority to investors and head management. If not wisely "aviation minded" managed then cases such as "discrimination" or "salary bargaining" or else of the same sort can quickly develop, a "mercenary" attitude settles amongst the employees and the whole thing drifts quickly down to the worms ....
btw ... hello GKaplan ...."long time no see" ))))