Jacob is setting up an argument here. Since he will be clearly stating (in verse 30) that there are conditions under which the Lord will permit polygamy, he is going to make it clear that this is not one of them, and that the special origin of the Nephites as a saved remnant from the destruction of Jerusalem makes of them a special people to the Lord, and that his laws are paramount for them. From a point of logic, Joseph is establishing his source for this restriction of polygamy as the Lord, and basing it on the calling out from Jerusalem. Thus Jacob will be able to construe law as only that law coming from the Lord.