User blog comment:B214/Volume 17 spoilers./@comment-24.2.168.254-20140220012350/@comment-16281343-20140220084416

It must have been labeled as forbidden since the previous possessors, which are humans, can't handle the power of Longinus Smasher. But Issei doesn't have a human's body anymore, so he can now handle powers that aren't possible for a human.