Considering the state of the world after July of 2000 with the breakdown of nations, the lack of government control (or ability to project that control),everyone trying to survive in the ruins of the world, I don't think anyone will actually care if someone is a mercenary or not. The attitude will probably be, "Who the hell can afford to be a mercenary because nobody has anything to pay them with".
The world has broken down to a situation where the government/authority/local warlord etc. etc. really only controls those lands they can actually deploy troops to. Very much like the city-states & local barons etc. etc. during the medieval era as mentioned by people in other threads. National borders are all but abandoned because nations as such just don't exist anymore. There are some areas here and there under the control of military or civilian remnants of the former nation but other areas are too devastated or too far to be worth controlling. Plus there's zones of poison/radiation, zones where it's a free-for-all fight for whatever is left inside - this all makes a patchwork of territories that would be fought over or fought through but leaving plenty of places for people or groups to hide out, travel through, go scavenging and so on.
I think in this environment, mercenaries can most definitely exist but in the sense of the Freelance Companies of the medieval era. Aside from the remnants of a nations military, these 'Freelancers' would probably be one of the few organized and competent (probably) military groups left and they just happen to hire themselves out to whoever could pay rather than support any one government/warlord etc. etc.
I don't think there'd be any government of significant power to prevent such groups from organizing let alone enforce any edicts about whether mercenaries should be considered marauders or not even if some of those mercenray groups take a few turns at being brigands or are hired by marauders. That also leaves open the classic scam by the robber barons and some mercenary companies in the medieval age - disguising themselves as brigands, attacking a town/travellers and then showing up a short time later to offer their services as protectors. Not only do they get some loot, they then get paid for some easy 'protection' duty for a threat that doesn't actually exist anymore.
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