User blog comment:IamJakuhoRaikoben/Chapter 471: Until the Battle is Done, Review./@comment-5303512-20160202131244/@comment-1776824-20160204000616

I'm pretty sure the purpose of Tenrou Island, as well as Future Rogue creating an entire magic to control dragons, is to show that Acnologia is still fundamentally human and is thus immune to the secondary effects of magics that specifically target dragons. Just my take, though.