Which word? The one where he tells His theocracy how to run their unique government? If we had a theocracy, then I'd agree that application of the dp for adultery etc was the right thing to do. Since we have something far, far removed from it, we cannot take those commands, which were given for the running a theocracy, as applicable to our republic.
Why not? We have, "Do not murder" and "Do not steal". Why not one more?
Legalism takes our eyes off of our Savior, off of our own need for that Savior, off of forgiveness and its benefits, off of mercy. It makes us see people as sinners instead of people in the bonds of sin. It makes it hard to love them or to impress upon them the beauty of redemption and love and mercy.
And the law should point to the saviour.