Albert Barnes Commentary 2 Kings 17:28

"So one of the priests whom they had carried away from Samaria came and dwelt in Beth-el, and taught them how they should fear Jehovah." — 2 Kings 17:28 (ASV)

The priest sent to the colonists was not a true priest of Yahweh, but one of those who had been involved in the calf-worship, probably at Bethel. Therefore, he would have been willing to tolerate the mixed religion, which a true priest of Yahweh would have condemned without compromise.