User blog comment:IamJakuhoRaikoben/Chapter 366: 1000 Souls, Review./@comment-7740542-20140110005529/@comment-7418318-20140110084821

Natsu was the one who defeated Jackal, so they might be interested in "recruiting" him like Minerva and Mirajane.

I guess they could always put some of those anti-magic shackles around his frozen arms and then let Silver un-freeze him. That way he'd be almost helpless and easy to force into the machine.