My guess: Because they are so god damn stupid its the only thing that is reliable. Because there are so many of them you need to trim off all the angry random men in the cause and you can't fire them. Possibly to breed the fear and suspicion of person in public places where your enemy goes. From the bombers perspective. Probably because the life there is so miserable and hopeless its preferable to get a large payout for your family or w/e they promise and check the fuck out Rather than live as a shooter or w/e returning to the squalor of your inbred desert life after a successful mission.