However, when you make a verb causative it becomes transitive, so you can use を. In fact, when you make an intransitive verb causative and you use を to mark the patient role (at least I believe it's the patient role; someone correct me on that if I'm wrong) it has a sense of coercion, and it gets translated as "make someone do something," but when に is used, that sense is lost and it gets translated as "let someone do something." For transitive verbs, however, when you make them causative you have to mark the patient role with に no matter the interpretation, due to the double-o constraint.