The amnesty bit is so that we weed down their numbers. As I have said before, if we "kill them all", we risk turning them into martyrs and creating generations of resentment in the region (e.g.: "The immortals are monsters! They massacred the poor Copperheads without remorse! Down with the Immortals!").
However, I do not propose that this stay there forever. No, we must set a deadline to this offer. And they must surrender to us and swear an oath of loyalty to England. After the deadline, anyone we capture is either conscripted if he's a minor footsoldier, or arrested if he's a major rebellion leader.