Korihor, after making one mistake by preaching in Jershon, follows it up with a second mistake—preaching in Gideon. Alma, during his first mission, chastised Zarahemla but praised the faithfulness of Gideon (Alma 7). Although the inhabitants of Gideon were Nephites, they had migrated from the land of Nephi after being converted from the apostate religion of Noah. I hypothesize that Korihor thought that their previous exposure to Lamanite beliefs would make it easy to persuade them to revert to those beliefs.
But once again, he misunderstood his target. These people, oppressed and forced to pay tribute to the Lamanites, had returned to faith in Yahweh and gospel as a prelude to escaping their captivity. Their personal experience with Yahweh had shown them the difference between the true religion and counterfeits. They had no mind to return to a false ideology.
Culture: Since Nephite law allows everyone freedom of belief, why do the inhabitants of both Jershon and Gideon bind Korihor and bring him before their judge. Were they not acting illegally?
While we do not understand the particulars of the Nephite law, we can understand the general principle. While Korihor had the right to preach his beliefs, the people of Gideon and Jershon had the right not to listen. It seems likely that the law allowed them to expel an undesirable person from their community, but they did not punish Korihor with torture or death for his beliefs.