Mercenaries in their purest and best form are a credible threat to United States democracy, as they provide a way for non-state sponsored personnel to threaten the government’s “monopoly of violence”. The monopoly of violence is the idea that the state is the ultimate judge and final decider of how violence is applied and where. They have the ability to punish individuals for using violence, and tell individuals which kinds of violence are permissible and which aren’t. Mercenaries obviously threaten this idea, but are still used in many different roles in US society.
If we steer away from the idea in popular culture of mercenaries and start to think less about the John Rabmos and the Expendables and start thinking more about the base definition of a mercenary, which is simply a soldier who’s only allegiance is money.
Using this definition, it’s easy to see examples of mercenaries all across the United States. The classic rent-a-cop, or otherwise hired security. These firms are private institutions who employ armed men and women who’s primary interest is their paycheck. So what makes the rent-a-cop OK but the Expendables illegal? Simply the fact that the rent-a-cop businesses aren’t good enough at what they do to outclass the government forces (military) and threaten the chain of command.
Maybe someday we'll have to outlaw rent-a-cops if they get too good at their jobs, but for the meantime, let's just all enjoy the crappy versions of mercenaries that exist within the bounds of the law.