I've been a lawyer my entire adult life--or, at least, what passes for a lawyer in the uncouth, rude, semi-savage region in which I practice. It may surprise you to learn I've appeared not only in what we call circuit or county courts, but in the appellate courts of this state. I've even, from time to time, put on my best suit, hitched horses to my wagon and traveled to the big cities to appear before the Federal District and Circuit Courts which hold sway here, where one might spend a lifetime searching for a spittoon and not find one. Truth to tell, I've even litigated matters involving constitutional and civil rights issues over the years.
I tell this sad story in the hope of explaining that I view the law as I think a practicing lawyer does; as a vast, growing, sometimes changing, mass of statutes, regulations, court decisions and decisions of administrative tribunals, enforcement mechanisms, penalties and civil liabilities the interaction with which rarely, if ever, involves what I consider moral or ethical questions. I therefore sympathize with philosophies of law which study the operation of the law as a working system, not as something which must comport with or must or does rely upon "true law" or "higher law" or "natural law." I find speculation along such lines to be largely irrelevant to the operation of the law, frankly, and its operation and its impact on our lives would seem to me to be its primary significance. How the law developed seems to me to be a question for historians; why we make laws may be something the social sciences can address.
So, when it comes to the relationship between morality and the law, I tend towards the position taken, for example, by O.W. Holmes, Jr. in
The Path of the Law. Holmes wasn't exactly a legal positivist, and is considered to be the father of American Legal Realism, but the view of legal positivism regarding morality and the law is similar. I tend toward that position based in part at least on my experience of clients who want to litigate expensively and endlessly in pursuit of justice, or the right or good, or over matters of principle, although warned that the result won't necessarily be just, right or good, because courts are courts of law, not justice, as Holmes once noted to someone appearing before him.
There will be instances where moral concerns motivate legislators or judges. I would maintain, though, that the law has expanded so much since the days in which Riggs v. Palmer was decided, and now impacts so many aspects of life, business, and government, that at least as far as judges are concerned, those considerations only rarely form the basis of a decision. Recourse may be made to such considerations in the absence of law, but there's so much law now that this is less and less necessary.
I've now read Riggs v. Palmer and am, of course, a better and wiser person as a result. But I haven't been so impressed by it that I'm persuaded that legal positivism (as I understand it) has been refuted, or even brought into question. It is after all a single decision, and I'd like to think that philosophers of law wouldn't consider it determinative, or even representative of the law as a system on that basis. Its holding was based on several grounds, including what I think are well established rules that in construing legislation the end in view is to determine the intent of the legislature, and that in construing wills courts should determine and follow the intent of the testator. Interestingly, this maxim regarding the interpretation of wills is such that the testator's intent is to be followed even if that intent is contrary to the "natural objects of his bounty" as I think the phrase is. So, courts will enforce wills even if they result in natural heirs receiving nothing, and the estate distributed to a local strip club, for example. I'm uncertain whether that rule has its basis in natural law.
So, one of the questions raised was whether, since the law of probate is intended to promote the orderly conveyance of property of a decedent in accordance with a will, it would be the intent of the legislature which adopted the governing statutes that a decedent's murderer would receive the estate. That of course would depend on the intent of the testator. Would a testator intend that his/her murderer receive the estate? Probably not. Is that judgment dependent on natural, or true, or higher law? The court then proceeds to consider matters more properly defined as based in morality.
I don't know how much it matters to the law, as an operating system, why laws are generally obeyed. I think it's more likely people obey them, when they do, because they believe the consequences of violating them--potential civil damages, criminal penalties, costs of defense--make it sensible to do so, not because they believe them to be based on a "higher law." But again, I think the "higher law" whatever it may be is less and less a factor as the law grows, and other considerations, economic and social play a greater part in the functioning of the legal system.