These individuals are no more mercenaries than is the guy who rattles door knobs in office buildings at night. Or, if they are, than so is everyone else who provides a service for a fee. Working as a guard for a company that is repairing damaged infrastructure is hardly the same thing as being a member of an organized armed force that is conducting offensive military operations, such as what occurred in Rhodesia 30-odd years ago. There are lots of articles out there, this
one is about two guards (neither US citizens) who were killed last week.