For two reasons, both of them spurious.
First, he had seen too many bad attempts to lay down natural law for human beings. A lot of people had made mistakes in deducing such law, and Mises may have thrown the baby out with the bathwater: he figured since so many alleged natural-law-givers had been wrong, there was no such thing as natural law.
But surely, there are vast disagreements among economists, as well. Just because Keynes, say, was wrong in his approach to economics does not mean that economics is hopeless.
Second, positive law is addressed from the legislative and judicial branch of the government to the police. By changing the behavior of the executive branch, it supplies incentives to the people. Positive law does not say: "You shall not kill"; it says: "If you kill, then you shall be punished."
But surely, some rights seem to be valid regardless of whether or not a violator of a right happens to be efficiently detected and prosecuted.
So, natural law is addressed to the individual, commanding him "not to kill," say. But each person decides for himself whether or not to kill. What purpose is there to a "command"? It seems entirely pointless. Ok, I hear you demand that I not kill. But who are you to tell me anything? "No one can tell another person how to live." You can alter my behavior by threatening to punish me for killing. That I understand. A command is entirely vain and nugatory. I decide whether to heed and obey any command.
I mean, a person cannot just bark orders at another. It's preposterous.
The first task for any expositor of natural law is to establish a person's ownership of his own body. But to say "I own my body" is already to sneak into the argument what it aims to establish. For "ownership" is a legal concept that presupposes the rightness of unfettered enjoyment of the services of the body and the wrongness of anyone's interfering with such enjoyment. What we have at the beginning rather is the primordial technological fact that I control my body. Not own it.
Our main endeavor then is to bridge the is-ought gap. For narrow happiness, the gap is bridged as follows: we say that if you want to attain end X (the "is"), then you ought to use means Y, Z, and W (the "ought"). But for human nature, this strategy is unavailable.
Very well, we note that I cannot relinquish my control over my body without dying; and it is impossible for anyone else to pick up or inherit that control from me. Smith cannot come to control Jones' dead body. But now consider the most basic injunction to man: that he ought to do good and avoid evil. An act of killing considered in itself destroys the victim, because he loses his highly valued control over his body (if it were not valued, then why has the victim not yet committed suicide? he does not have to wait to be killed), yet benefits not at all the killer, who cannot steal that control away from the person he kills.
As a result, killing simpliciter is an unmitigated evil that is not balanced at all with any kind of good. But if man is to avoid evil in general, this particular evil, too, ought to be avoided. Hence, not killing is a duty, and not being killed is a human natural right.
This right can be codified if we pronounce that any man "owns" his body with all the privileges of such ownership.
Mises did not think that ethics was a branch of knowledge at all. He did not consider it to be a valid discipline in which truth and falsity can be established. Ethics for him was neither science nor philosophy; it was nonsense. But here I think he was wrong.