Although the Celestial Thearch was an Emperor and the strongest in the Spiritual Realm, he still had to obey heaven's will.
Even with the ability to refine origin power and not become corrupted, the Mental Demon Battlefield was still sealed away for the Spiritual Realm to slowly feed on.
However, the circumstances were clearly different now.
The seal on Mental Demon Battlefield had been broken, and Demon Gods were appearing. To the Celestial Thearch, this was both a risk and an opportunity.
This was because the Spiritual Realm was unable to immediately absorb the Mental Demon Battlefield. As such, he was able to provide assistance and enjoy a share of the resources.
This was similar to the proverb: "A general in the field is not bound by the orders from his sovereign."