Robert Ingram illustrates the distinction quite clearly [my italics]:
... Due process of the law means a proper and just conviction of a crime. No one may be executed, imprisoned or fined unless he is a convicted criminal. At the time of the adoption of this Constitution there was none of the present day confusion about crime which sees any infraction of statute regulations somehow as criminal. But the real criminal law, which is summed up, I say, in the Ten Commandments, is such that, together with universal principles of justice, many if not all these regulations are unlawful; they are themselves enforced only by criminal acts.Of course, it should likewise be stated that, where the civil magistrate acknowledges and honors God’s law, “illegal” does not conflict with “unlawful.”
The Prohibition Amendment and the Volstead Act which made it operative furnish a clear illustration. The common law rights are guaranteed by laws that would punish anyone who takes unlawfully what belongs to another. Thus it is unlawful even for police authorities to put a man under arrest without either a warrant or personal immediate knowledge of a criminal act. It is therefore unlawful to search his person or his property apart from lawful arrest. Now there was no way to enforce the Volstead Act, which outlawed the possession of alcoholic beverages and also their manufacture, except by unlawful search and seizure.
If the Volstead Act was to be enforced, the authorities then had to break the law and violate the common law rights of every one. The result was an intuitive recognition by the general public of the fact that they no longer enjoyed the protection of the law (which is also known as the protection of God since it is God’s law): therefore they turned almost overnight to the protection racket — to gangsterism. The notorious lawlessness of the Prohibition era is quite wrongly explained as the consequence of men becoming accustomed to breaking the law by buying bootleg whiskey; it was simply the collapse of the common law caused when the power of government was used unlawfully.
Attempts to outlaw the possession and sale of narcotics today present the same problem. This is no argument for permitting unlimited use of narcotics; it simply is a recognition of the very precise and serious limitations facing rightful control. Perhaps all that can be done is to follow the ancient precedent used to deal with prostitution — outlaw solicitation. Fornication itself is unlawful under the common law (based on the commandment forbidding idolatry), but raiding a house of prostitution is highly questionable. On the other hand, solicitation, or making the offer to fornicate for a price, is necessarily an open and observable act and is unlawful. So narcotics might be controlled lawfully* by outlawing offering them for sale. No one would expect by these measures to “eliminate” either prostitution or traffic in and use of narcotics; but they probably would serve the common good by keeping these things within tolerable bounds.
The operative principle is to be noticed carefully. The usual common law rights are simply the reverse side of common law prohibitions. It is unlawful to take any of this world’s goods from a man — his person, his liberty, his family, or his goods. It is unlawful for the sheriff or the king to do so. Under the law, then, a man has a “right” to his life, his limbs, his liberty and his property simply because it is wrong to take them from him except in just punishment for breaking that same law. The law is a closed circle, a complete fence. Within it men are free and have innumerable “rights” if one wants to think of them that way. But these rights appear from the wrongs specified by the common law.
~ T. Robert Ingram, What’s Wrong With Human Rights (free PDF available here), pp. 53-54
* The book’s original text reads, “So narcotics might be controlled unlawfully by outlawing offering them for sale.” But given the context and Ingram’s line of reasoning, I think that “unlawfully” is a printing/publisher’s error, which I have taken the liberty of correcting here.