I think you miss the point entirely. A witness in the context of the Law is an accuser, not an observer. And a righteous servant of God would never bring a witness against the Lord's Anointed. It is God who judges His Anointed, not you or Post. Well, it shouldn't be you and Post, but it is.
I think you are trying to say God didn't follow His own law here, so you can have an excuse to disregard whatever Law of God that doesn't fit your religious lifestyle. And I think it is important to note EVERY WORD of God in His Law before you accuse God of showing partiality for David, or before you try and convince people that the Christ taught against His Fathers Commandments.
I can only go by what the Scriptures say, so if you have a religion which rejects or omits the following Word's of God I can only advise you to reconsider the religion.
Lev. 26:
33 And I will scatter you among the heathen, and will draw out a sword after you: and your land shall be desolate, and your cities waste.
34 Then shall the land enjoy her sabbaths, as long as it lieth desolate, and ye
be in your enemies' land;
even then shall the land rest,
and enjoy her sabbaths.
35 As long as it lieth desolate it shall rest;
because it did not rest in your sabbaths,
when ye dwelt upon it.
36 And
upon them that are left alive of you I will send a faintness into their hearts in the lands of their enemies; and the sound of a shaken leaf shall chase them; and they shall flee, as fleeing from a sword; and they shall fall when none pursueth.
37 And they shall fall one upon another, as it were before a sword, when none pursueth:
and ye shall have no power to stand before your enemies. (David being carried away by his flesh)
38 And ye shall perish among the heathen, and the land of your enemies shall eat you up.
39 And they that are left of you shall pine away in their iniquity in your enemies' lands; and also in the iniquities of their fathers shall they pine away with them.
40 If they shall confess their iniquity, and the iniquity of their fathers, with their trespass which they trespassed against me, and that also they have walked contrary unto me;
41 And
that I also have walked contrary unto them, and have brought them into the land of their enemies;
if then their uncircumcised hearts be humbled,
and they then accept of the punishment of their iniquity:
42 Then will I remember my covenant with Jacob, and also my covenant with Isaac, and also my covenant with Abraham will I remember; and I will remember the land.
This is just as much a part of God's Law as "Thou shall not kill". David was a living manifestation of God's Promises here, the curses and the blessings.
God did not ignore His Law here, He established it.