The fact that they hadn't done so yet made their intentions clear.
If Orion refused their request, it would appear as though he lacked confidence in his own strength, was afraid to make enemies, and was indifferent to the sacrifices they had made to defend this land.
That kind of impression would sow doubt, weaken his influence, and embolden others to stir conflict after the oath ended.
It would also discourage those who currently submitted under his overwhelming presence, as they would feel there was nothing to gain from following him.
If he agreed, he would be signing up to go against any enemy they claimed had attacked them.
Though this would earn him a reputable standing and build good relations with those present, it would also cause Paradise's advances in the Spirit Realm to be met with more resistance.
And if a battle broke out, resulting in their defeat, the Spirit Beast Coalition would no longer hesitate to launch a war against them.