The crux of libertarianism is not the non-aggression principle, but proportional punishment. It is important for libertarians to stress this in order to properly answer some of the major critiques people throw against anarchy (I use the words libertarian and anarchy as synonyms). The reason why libertarians only believe in using force against those who initiate force is because to go above this would be excessive, and hence a violation of proportional punishment. The punishment should fit the crime and if someone uses a small amount of force the punishment will be proportional. No principled anarchist would advocate incarceration for stealing a person’s socks or even steal a thousand dollars. No principled anarchist would advocate the death penalty for crimes where the result is not death. Just people do not advocate chopping the hands off of thieves, even when we understand theft should be against the law.
The reason it is important for libertarians to stress proportional punishment more than non-aggression (in my opinion) is to be able to answer the hard questions, such as “would you steal an apple to live?” The non-aggression principle does not properly answer this question since no honest person would let himself starve to death over stealing a stranger’s apple. But proportional punishment theory does properly answer the question and the answer is: According to proportional punishment theory, the penalty for stealing an apple would not be jail or death but having to reimburse the person you stole the apple from. The penalty might be two apples or three for stealing one but it would be less harsh than what would result from starving to death. Doing a simple cost-benefit analysis tells one to steal the apple and then take the proportional punishment which comes with it. Libertarianism is not really concerned with non-aggression per say but proportional punishment. The only reason to appose aggression and the reason behind the non-aggression principle is that it violates proportional punishment.
Anarchy/Libertarianism is About Proportional Punishment, Not Non-Aggression