ISIL uses it Islamic religious bigotry to force people to fund its operations

Rubin Center:

The concept of repentance (tawba in Arabic) is key for the Islamic State (IS) whenever it takes over a new area and aims to consolidate control. Whoever undergoes repentance under IS successfully is issued arepentance ID document that stipulates the person has abandoned his/her apostasy and is to be treated like the rest of the Muslim populace under IS rule (Specimen 3V). The repentance process has two main applications. First, it is used to subdue people who had been affliated with rival forces primarily through the quid pro quo principle of sparing one’s life in exchange for surrendering all weapons to IS. This generally includes members of Syrian rebel groups, Sunni conscripts to the Syrian army, and members of the Iraqi army and police. For a sample case, see Specimen 1B in the archive of IS admin documents with the case of the Albukamal area in Syria. At times, IS has faced agitation against its rule from those who had repented, such as in the Fallujah area, which led to the tightening of repentance conditions last year (Specimen 6Q).

Repentance is also important for co-opting existing administrative structures as far as possible into IS governance, as many who repent will have worked in jobs that cannot be applied to the IS system and so will be left without actual work (leaving aside salary payments for the moment). Most notable with regards to co-optation within the strictures of IS governance is the attempt to bring the Iraqi and Syrian education systems in areas of IS control in line with IS ideology, which entails securing the repentance of teachers and education admin staff and then subjecting them to ‘re-education’ in the form of Shari’a lessons, using IS textbooks like al-Taqrirat al-Mufida fi Aham Abwab al-Aqida (Beneficial Courses in the Most Important Aspects of Doctrine).
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There is more.

ISIL charges a fee for "repentance."  It is another aspect of the religious nature of their regime that the Obama administration refuses to acknowledge.  Because it will not accept that ti is a religious movement its efforts to defeat it have been largely unsuccessful.