"...they are not categorized as mercenaries, they are categorized as contract workers."

But all mercenaries are contract workers. That's like saying "This can't be categorized as a banana, it has to be categorized as a fruit."

It all depends what sort of work they are contracted to do - and who is doing the categorizing, of course.

Whether someone employed to fight is a mercenary, a volunteer, or a conscript, that in itself should make no difference to the way they are treated. That's why the invention of the term "illegal combatant", with a reduced right to humane and civilised treatmnent when captured, is a step backwards. If a combatant is alleged to have done something illegal, such as massacre civilians or torture prisoners, it doesn't make any difference what kind of combatant they are - but dealing with that is a matter for an appropriate court of law, not for the people holding them captive.