The following is an excerpt from Walter Wink’s book, The Powers that Be: Theology for a New Millennium. (emphasis mine)
Why carry the soldier’s pack a second mile? Does this not go to the opposite extreme by aiding and abetting the enemy? Not at all. The question here, as in the two previous instances, is how the oppressed can recover the initiative and assert their human dignity in a situation that cannot for the time being be changed. The rules are Caesar’s, but how one responds to the rules is God’s, and Caesar has no power over that.
Imagine, then, the soldier’s surprise when, at the next mile marker, he reluctantly reaches to assume his pack, and the civilian says, “Oh, no, let me carry it another mile.” Why would he want to do that? What is he up to? Normally, soldiers have to coerce people to carry their packs, but this Jew does so cheerfully, and will not stop’. Is this a provocation? Is he insulting the legionnaire’s strength? Being kind? Trying to get him disciplined for seeming to violate the rules of impressment? Will this civilian file a complaint? Create trouble? From a situation of servile impressment, the oppressed have once more seized the initiative. They have taken back the power of choice. They have thrown the soldier off balance by depriving him of the predictability of his victim’s response. He has never dealt with such a problem before. Now he must make a decision for which nothing in his previous experience has prepared him. If he has enjoyed feeling superior to the vanquished, he will not enjoy it today. Imagine a Roman infantryman pleading with a Jew to give back his pack! The humor of this scene may have escaped us, but it could scarcely have been lost on Jesus’ hearers, who must have been delighted at the prospect of thus discomfiting their oppressors.
Jesus does not encourage Jews to walk a second mile in order to build up merit in heaven, or to be pious, or to kill the soldier with kindness. He is helping an oppressed people find a way to protest and neutralize an onerous practice despised throughout the empire. He is not giving a nonpolitical message of spiritual world transcendence. He is formulating a worldly spirituality in which the people at the bottom of society or under the thumb of imperial power learn to recover their humanity.