Timothy Sandefur has two great posts on 14th Amendment incorporation here and here. Both are well worth reading, especially if you are a libertarian. I have to admit, when I first taught philosophy of law, I did give my students a version of what Sandefur calls "vulgar incorporation." Partly this was because it was easier to teach that way; partly it was because, at the time, I didn't really understand incorporation myself (and I'm still no expert, as this post will likely indicate.)
Here is the relevant portion of the 14th Amendment:
"No state shall...deprive any person of life, liberty, or property, without due process of law."
The question is: what does due process of law require?
When I was teaching philosophy of law for the first time, I was very much taken with Justice Scalia's criticism (in his book, A Matter of Interpretation) of what is often pejoratively called substantive due process. For Scalia, all that due process of law requires of a state is that if it is going to deprive a person of liberty, it has to go through the right process, i.e. it has to pass a law, give the person who supposedly violated the law a trial, and so on.
As Scalia says (p. 24), "Property can be taken by the state; liberty can be taken; even life can be taken; but not without the process our traditions require -- notably, a validly enacted law and a fair trial." We could call Scalia's interpretation of the Due Process Clause "procedural due process."
This is not an unreasonable way of interpreting the Clause, but it is certainly not the only way to interpret it.
In contrast, the substantive due process interpretation of the Clause requires the states not only adhere to the right process, but also that the laws they pass by way of that process meet additional, more-than-formal requirements. According to the vulgar form of incorporation, these are the requirements set out in (most of) the Bill of Rights.
In other words, the idea is that the term "liberty" in the Due Process Clause of the 14th Amendment includes or incorporates within it at least some of the guarantees set out in the Bill of Rights (like the 1st Amendment), making it possible for these guarantees to be applied not only to federal action, but to state action as well. If California throws someone in jail for expressing a political opinion, federal courts would free him (and strike down California law) on 1st and 14th Amendment grounds.
Cohen v. California (1971) demonstrates this nicely. In his opinion, Justice John Marshall Harlan II wrote that "The state may not, consistently with the First and Fourteenth Amendments, make the simple public display of this single four-letter expletive a criminal offense."
As Sandefur notes, the "liberty" guaranteed in the Due Process Clause is not in a one-to-one relationship with the Bill of Rights. When a federal court is assessing the constitutionality of a state law, it is not simply a matter of determining whether, under the Constitution, the federal government would be permitted to make such a law. Things are a little more complex than that (or should be, at least.)
Incorporation, therefore, is not an exact one-to-one ordering of state and federal law; it is an inquiry into whether the state’s action so exceeds the standards of “lawfulness” as to violate the due process of law clause—or whether it deprives a person of the privileges or immunities of citizenship—and that inquiry is guided by reference to the Bill of Rights, among other things, which lay out the standards for determining what constitutes “due process of law.”First emphasis added. Now it's an important to ask: just what are the standards of "lawfulness"? What criteria must a "law" meet in order to be reasonably called a law? Is it just a matter of the law being written down in the right place?
I bored my students to death, I'm sure, but we did examine legal positivism, especially Austin's relatively crude version of the doctrine. For Austin (basically), a law is just a law if it is backed up by a powerful enough authority. Austin's criteria for "lawfulness" is "enforced-ness."
Thus, if we are legal positivists of this stripe, we will likely find Scalia's interpretation of the Due Process Clause fairly congenial. As long as California could enforce it, the hypothetical California law we are considering would meet the standards for lawfulness, and -- on due process grounds, at least -- the federal court could not object to it.
As I understand Sandefur's position, his claim is that a law must meet other, perhaps moral standards, if it is to be rightly considered a law. The argument seems fairly tight: if California meets the procedural standards for the making of a law, but does not meet these other, substantive standards, then it hasn't made a law at all. Thus, when it puts people in jail on the basis of that "law" it is depriving them of their liberty without due process. It would, in fact, be exercising naked coercion, might without right.
How do we know what the moral standards of lawfulness are? As Sandefur notes, answering this question requires an engagement with political philosophy. Some conservatives (like Scalia) see such an engagement as just an excuse for judges to ignore the law and impose their own views. That's certainly a danger. It's even more of a danger if one does not believe in moral truth at all.
But the important point is that the Framers of the Constitution did seem to believe in moral truth. The 8th Amendment (to take an example of Ronald Dworkin's) seems to lay down an abstract principle, rather than a specific prohibition of certain forms of punishment. We do not, I think, travel far from their vision in thinking that a concern with moral and political philosophy is essential to a proper interpretation of the Constitution.