Even though he had the opportunity to impose an oath on them, he didn't bother with it. This was because he had lost all faith in the sanctity of oaths.
Just last timeline, he was backstabbed and put in a deadly position after the Dao Lords promised not to attack him or sabotage his efforts to expand his Domain in the Sea of Worlds.
The loophole they used was that they didn't attack him personally, and they didn't sabotage any attempt to expand his Domain since the Domain they were responsible for removing was already established. In fact, what they did couldn't count as sabotaging since he was always going to lose the Domain in the Blessed Heaven World anyway.
The Domain was supposed to be lost when the Blessed Heaven World ascended. He knew about this and was satisfied with it. In fact, he had made a deal with them just to earn that satisfaction from him. So what they had done was to help him hasten the loss of the Domain.
