This is more of a question than a comment. I haven't played more of BoA than the tutorial, an 1813 campaign solo, and half of a 1781 Cornwallis PBEM, so I don't know whether my experience is typical or extreme. But I'll ask anyway.
In my last game turn, a rebel force, eight companies, was caught in the open by a British force, twelve companies. They were soundly defeated, which makes perfect sense in my book. However, they were also annihilated, killed (or captured?) to the last man. Two companies were lost to ranged fire, and then six in melee. That seems a bit extreme to me. I can picture a rebel army dissolving in panic after being soundly whipped by a superior force of redcoats, but I can't quite see them disappear from the face of the earth entirely. Even if the force were temporarily hors de combat after such a battle, surely at the end of the month (= turn) a good deal of them would have returned to the colours?
So, are defeated forces usually eliminated completely in BoA, and if so, does it make sense?