After the aside about the Lamanites, Mormon returns to the Amlicites. That return highlights the reason that he inserted the information about the curse on the Lamanites. The marking of the Amlicites became a symbolic indication that they had mingled with the Lamanites, and perhaps that they believed something of the traditions of the Lamanites. Thus, they were also cursed, and the mark was the symbolic indication and acceptance of that cursing.
By mingling with the Lamanites, the Amlicites were no more Nephites. “And again, I say he that departeth from thee shall no more be called thy seed.” After that statement, it is also clear that such people who left the Nephites were no longer covered by the Nephite promise of the land.