The officers may have been allowed so as to make the fort a better settlement and let them live there longer.
Step-by-step explanation:
Fort Union had always been fairly busy and it required a lot of food supplies due to the amount of the refugees coming in at all times.
This could help the pockets of the officers who could just now have farms there and use their own produce to turn them profits and have readily available sources of food for the migrants landing there.
This allowed them to have a rather stable stay at their post as they would in this time.