They are acting in complete 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)
The Islamic State group has set a demand of a $30 million ransom for the release of hundreds of Christian hostages in their captivity in Syria.
The terrorist organization is demanding up to $100,000 per individual, according to a report over the weekend from Fox News. The group captured between 250-300 Christians on February 23 in the Northern Hasaka province of Syria, an area native to the country’s Christian population.