[Ruling on What Rebels, Khārijites, and Oppressors Take from the Muslims]

The answer: If the one fighting the believers is from the people of jabr (compulsion) or tashbīh (anthropomorphism) or those similar to them among those who are judged to be disbelievers, then by analogy they own what they have taken and prevailed over, by analogy with the polytheists, due to their sharing in disbelief.
What supports this is that Amīr al-Mu’minīn (Peace be upon him) did not hold the people of Baṣrah liable for what they had destroyed of the Muslims’ property.
If it is said: Amīr al-Mu’minīn (Peace be upon him) did not judge them with the ruling of disbelievers; rather, it is narrated from him that “the land is a land of Islam, and it is not permissible from their property except what they have brought forth for war; as for what is beyond that, it is forbidden, because the land of Islam makes what is in it forbidden.”
We say: The war of ʿAlī (Peace be upon him) is the war of Allah and His Messenger (May Allah bless him and his family and grant them peace), and whoever wages war against Allah and His Messenger is a disbeliever.
What further strengthens this is what is narrated of the exchange between ʿĀʾishah and al-Ashṭar, when she said to him regarding the people of Jamal: “Do you not know that the blood of a Muslim is not permissible except in one of three cases…” – the ḥadīth – so al-Ashṭar said: “For some of that we fought them, O Mother of the Believers.”
Moreover, it is something agreed upon that deeming it lawful to kill a believer is disbelief and an exit from Islam. And it is known that the people of Jamal, outwardly, were deeming it lawful to kill the companions of ʿAlī (Peace be upon him) and those obedient to him. And the ummah today is unanimous that Amīr al-Mu’minīn (Peace be upon him) and those obedient to him in the Battle of Jamal were the party of truth.
Once you understand this, (you know that) Amīr al-Mu’minīn (Peace be upon him) did not judge the land with the ruling of the land of shirk for one of two reasons:
1. Either because the doubt (shubhah) was strong, due to the presence of ʿĀʾishah, the wife of the Prophet (May Allah bless him and his family and grant them peace), and Ṭalḥah and al-Zubayr, who were among the greatest of the Companions, so the general people of Baṣrah were deluded by them.
2. Or out of consideration by Amīr al-Mu’minīn (Peace be upon him) for the general interest; for if he had treated them as he treated the polytheists, perhaps some of those obedient to him would have wavered from him, and his enemies would have found a way thereby to blame him and disparage him. What supports this is that when, after Jamal, some things occurred from the people of Baṣrah, Amīr al-Mu’minīn (Peace be upon him) bestowed favour upon them by pardoning them, which indicates that he left off some of what they deserved of punishment.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2