It's because Mohammed did so, too! Both Bouyeri and the London bombers lived on state support. According to Buzzmachine, who in turn quotes the Sunday Times, this kind of behavior is completely halal:
Instead, the young follower, Nasser, who receives £44 job seekers’ allowance a week, said it was permissible to “live off benefits”, just as the prophet Mohammed had lived off the state while attacking it at the same time. Even paying car insurance was seen as supporting the system. “All the (Saviour Sect) brothers drive without insurance,” he said.