What about instances, where there is no petitioner?
I think Fr. Amorth alludes to the fact the God may, in some cases, allow the Devil to attack someone who has not petitioned evil spirit or had a curse put on them. Why, we cannot know. Maybe to test our faith, ordeepen our faith, or, in final victory over evil, to increase people's belief in the mercy of God. In Felicitas Goodman's book "The Exorcism of Anneleise Michelle" one sees that situation exactly. A devout, pious Catholic girl mercilessly attacked (and eventually killed) by an evil spirit.