Castiel was not welcome in the current matter. He had the potential to complicate the plan.
There was already much in the works to keep his attention focused elsewhere, but clearly he had slipped through the cracks at the moment. Which meant that it was better to give the appearance of a victory at the moment. There would be time later to pick up where they'd left off. Time when hopefully the angel would be otherwise engaged. Might as well make it look convincing in the meantime.
Jo's eyes turned black for just a moment. And then they returned to her usual brown as the presence released its hold on her mind. She looked at Cas in surprise and then crumpled.