User blog comment:B214/Question and Answers 6/@comment-24098614-20161212223522/@comment-26821288-20161213090629

I think they didn't steal souls, or made Humans give up their souls, really, but actually, the payment for the wishes was the Human's soul. Remember when Issei visited Azazel for the first time? Azazel didn't have anything to pay for Issei, so he said it'll need to be either his soul or that expensive draw. So I think that maybe paying with your soul was normal back then, and now it's not too normal, but maybe, under certain circumstances, it is possible and okay to do so.