Even more accurately, there is no sacred text that says, "If, in a few centuries, a new faith arises based upon the teachings of my Son, whose coming will NOT be the arrival of the Messiah you have heard through my prophets, then the followers of the new faith shall display these tablets in their courts of law as a sign that these tablets are the source of their laws."
In fact, I can't really think of a command in the Bible to display religious symbols in civil institutions. Or not to. So whether or not it is right or wrong is left open to some interpretation. At this point, however, whether it is legal or illegal is not.
Roy Moore believes that all law comes from God - that is exactly what William Blackstone believed and Blackstone's legal philosophy was ascribed to by our founders. The 10 commandments are indeed the foundation for all moral and civil law. The 10 commandments embody absolute moral principles - lying, stealing, murder are wrong all the time for all time. It starts there. Mankind isn't and the State isn't.
Even more accurately, there is no sacred text that says, "If, in a few centuries, a new faith arises based upon the teachings of my Son, whose coming will NOT be the arrival of the Messiah you have heard through my prophets, then the followers of the new faith shall display these tablets in their courts of law as a sign that these tablets are the source of their laws."
Since God is eternal (notice our founders embraced monotheism), his moral precepts are also eternal as they flow directly from His character. So, lying was wrong before Moses, after Moses and still is. It's universal and transcendant truth.
There is one possible example in scripture about display. The Ark of the Covenant contained the tablets of the law and it was always carried before the people and was eventually placed in the temple in the Holy of Holies. Moore is right about one thing. The 10 commandments are the basis of U.S. law. All one has to do is read the founders' writings to know that. So, if they are the basis of the origin of our laws, what's the problem? There is only a problem if one believes that law evolves in the atheistic darwinian sense (here is that religious connection again!).