Let's assume they have, say, two actions to use. They can either Main War Party to protect what they have and thus make any attempt to retake it very difficult, or do a secondary Settle and secondary War Party, and gamble that the War Party does well enough, and that the enemy doesn't try very hard to retake it while they make the settlement. That's a bad gamble to take, and would be pretty likely to fail barring the intervention of luck on their part.
Whereas if we do the settlement action for them, they can devote their actions to defense this turn, and get the buff from the settlement without sacrificing any of their power.