Otherwise, the non-aggression principle is entirely formal and lacks any content. It presupposes a theory of violence which in turn requires a theory of property and its just and unjust possession and ways of acquisition.
Suppose one says: nobody has a right to the full fruits of his labor; other people have rights to your gains from trade.
Your justly owned property is precisely what the government has elected to leave you with.
It is true that all property comes into existence controlled by some private individual, but that control is illegal or unjust. The government then seizes that unjustly held property which has always been legitimately its own. Thus, our social system may be summed with: "the government is a legitimate institution, and whatever it does is legitimate."
We can define into existence any number of such systems, even socialism: the government owns all goods, including the labor of the citizens of which it has the right to dispose as it sees fit. Instead of "finders keepers" or "first to control" or "first to mix labor with," the principle of just acquisition is the right of government possession. Regardless of who was the first possessor, the government has the right to despoil him and take his control away. Whatever the state does is automatically right, while not so for private individuals.
Now of course it will be objected that this theory of property is altogether wrong. And I agree. But the point is that one must first propound a correct theory, and then say that within its confines one ought not to initiate force. For the libertarian system, such as Rothbardian natural law, also uses violence against unjust claimants to property. Any property can and must accrue to its legitimate owner; it's just that who the legitimate owner is will be different from one property system to another.
In other words, under socialism, say, non-initiation of force may be exceedingly perverse -- a slave principle. In fact, in that system, in order to survive, a person usually cannot avoid stealing from the state, i.e., privatizing some property. This kind of theft may well be judged perfectly salutary from the libertarian perspective; and, on the contrary, non-initiation of force may be likened to a principle of "surrendering your birthright to a self-appointed elite of supermen" and to "abandonment of self-determination and to yielding to other people's domination," in Mises' words (HA, 878), that is, to slavish obedience to those who would without qualms control every aspect of one's life.