The priest sent to the colonists was not a true Yahweh-priest, but one of those who had been attached to the calf-worship, probably at Bethel. Hence, he would be willing to tolerate the mixed religion, which a true Yahweh-priest would have unsparingly condemned.
Related Commentaries of 2 Kings 17:28
2 Kings 17:28
28 Then one of the priests whom they had carried away from Samaria came and dwelt in Bethel, and taught them how they should fear the LORD.