Ah mouseferatu, while you may be correct in some aspects of your analysis, I think the main key is that the DM is overworked, trying to efficiently judge the whole outcome, and make sensible decisions for 5 combatants, + any creatures standing by, while each of the PCs has but one thing to do.
But the main purpose is to give the PCs a challenge, not to kill them.
So if the band of anti-PCs work for the opposite team, or are employed to hunt down the same missing merchant by another faction, or just plain claim the credit for the PCs victories. But like PCs, these guys want to live, and they always have resources to escape, or bribe their way out of trouble.
Every battle does not need to be to the death, sometimes it is just enough to hinder the PCs a round or three, and then withdraw. Especially if the anti-PCs have lots of healing resources.