The people of Ammon had been given the land of Jershon as an inheritance because it was inside Nephite territory. That plan of geographic protection was now destroyed because of the defection of the Zoramites in Antionum. Lamanites in the wilderness may indicate that they were not beholding to a larger city, but when they were combined with the Zoramites there was now a greater threat. Now the buffer zone is removed, and the perhaps unorganized and therefore less threatening “wilderness” Lamanites are now associated with a city, and an organization. Jershon is no longer a haven, but in immeditate danger.
The result is that the people of Ammon leave the land of Jershon and come over to Melek. They leave the land of Jershon to the armies of the Nephites. There are two things that we should understand from this passage. The first is that the armies of the Nephites are going to require sustenance, and that sustenance comes from the land. In most wars, the people stay in place to support the army. In this case, we have the people of Ammon who have vowed to avoid taking up arms. For their safety and protection they are moved to a safer location.
The second thing that we must understand is that the land of Jershon would not have been abandoned entirely. Some peoples who had not take the vow of the people of Ammon would have remained to support the armies, and it is possible that some of the armies would be assigned to maintain the farmlands that were abandoned by the people of Ammon. The maintenance of the production capacity of those lands would be essential to support the armies.