I think the answer is quite straightforward: being passive beneficiary of the spouse's crime makes one culpable, especially since he did nothing to remedy / repent of the crime and since as King he was supposed to be the upholder of justice.
First, note the following points from the story as recounted in 1 King 21:
- Ahab made known to Jezebel his desire for Naboth's vineyard and knew that Jezebel is going to help him (v 7)
- The letters were in Ahab's name (v 8)
- Ahab knew about the crime afterwards (v 15)
- Ahab profited from Jezebel's crime, so implicitly agreed with the crime instead of repenting to God (v 16)
- Jezebel was also punished by death (v 23)
Then note how as a King he was supposed to uphold justice, especially in regards with the innocent, a job description frequently mentioned in the prophetic books. For example, Jer 22:1-5:
A Message for Judah's Kings
...
3 This is what the Lord says: Be fair-minded and just. Do what is right! Help those who have been robbed; rescue them from their oppressors. Quit your evil deeds! Do not mistreat foreigners, orphans, and widows. Stop murdering the innocent!
Ahab, then, was fit to be punished, because he implicitly murdered the innocent. The charge (v 17) and punishment (v 18-19) is similar to that of Jehoiakim. See Jer 22:17-19:
17 “But you! You have eyes only for greed and dishonesty!
You murder the innocent, oppress the poor, and reign ruthlessly.”
18 Therefore, this is what the Lord says about Jehoiakim, son of King Josiah:
“The people will not mourn for him, crying to one another,
‘Alas, my brother! Alas, my sister!’
His subjects will not mourn for him, crying,
‘Alas, our master is dead! Alas, his splendor is gone!’
He will be buried like a dead donkey—
dragged out of Jerusalem and dumped outside the gates!