Albert Barnes Commentary 1 Kings 21:26

"And he did very abominably in following idols, according to all that the Amorites did, whom Jehovah cast out before the children of Israel.)" — 1 Kings 21:26 (ASV)

The Amorites appear here as representatives of the old Canaanite nations (Genesis 15:16 note). It seems to be implied here that their idolatries were essentially identical to those of the Phoenicians, which Ahab had adopted.