They’re acting in full accord with Islamic law. Here is a salient passage on this issue from a Shafi’i manual of Islamic law:
When an adult male is taken captive, the caliph considers the interests … (of Islam and the Muslims) and decides between the prisoner’s death, slavery, release without paying anything, or ransoming himself in exchange for money or for a Muslim captive held by the enemy. (Umdat al-Salik o9.14)
A revered Islamic jurist, Al-Mawardi, agrees with Umdat al-Salik:
As for the captives, the amir has the choice of taking the most beneficial action of four possibilities: the first, to put them to death by cutting their necks; the second, to enslave them and apply the laws of slavery regarding their sale or manumission; the third, to ransom them in exchange for goods or prisoners; and fourth, to show favor to them and pardon them. (Al-Ahkam As-Sultaniyyah (The Laws of Islamic Governance), 4.5)
“ISIS Demands $23 Million for Assyrian Hostages, Vigil Held in Australia,” AINA, May 1, 2015 (thanks to The Religion of Peace):
(AINA) — Assyrian church leader are reporting that ISIS has rejected a ransom offer for the 230 Assyrians it captured on February 23 in the Hasaka province of Syria (AINA 2015-02-23). 253 Assyrians were captured in the initial attacks on the 35 Assyrian villages on the Khabur river. 23 were subsequently released but 230 remain in captivity, including 52 children, 84 females and 95 males.
ISIS has demanded 100,000 US dollars for each hostage, for a total of 23 million.
Leading the negotiations with ISIS is Bishop Mellis of Australia. In an interview with SBS Radio in Australia, Bishop Mellis said:
We are a poor nation. These people have not done anything wrong and won’t harm anyone. We as Assyrians do not have this amount of money you are asking for.
We offered an amount of money that we cannot disclose at this time. With the amount we offered, we thought it was acceptable, to have the return of the 230 people
After two days, they (ISIS) told us, “the amount the church offered was not acceptable. From now on, we will no longer negotiate with you.
The hostages have been moved to Raqqa, the ISIS stronghold in Syria, and are now awaiting trial in Islamic court under Sharia law, where a Muslim judge will decide their fate. Desperate church leaders have pleaded for assistance neighboring countries, including Turkey, in securing the release of the Assyrian hostages.