Mercenaries are soldiers hired to fight for another military. Most military bodies require soldiers to take some sort of oath of service. To take an oath of allegiance to one military and then to fight for another is not only a conflict of interest but a violation of the oath. Therefore, mercenaries are not consistent with the law.
Not only do mercenary soldiers raise legitimate concerns of loyalty to the nation they are employed by, it provides a direct corporate line to war. Companies that are not federally regulated and maintained should have no involvement on the battlefield, at least not in such a manner. Mercenaries are dangerous, unjustifiable, and an untrustworthy agent to be representing and defending the nation's interests.