User blog comment:IamJakuhoRaikoben/Chapter 371: Tartarus Arc: Part 2 - Song of the Heavenly Dragon, Review./@comment-3424593-20140214155827/@comment-7418318-20140214215244

It makes sense. Even if they weren't a danger to the Demons themselves, the Fairy Tail Mages with their number and reputation for destroying stuff might still destroy a lot of valuable equipment and ressources, like the tanks in which they revive fallen Demons and transform "new children".

And after Tempesta and Jackal were defeated by Fairy Tail Mages, they can't be sure that the Humans are no danger to them, especially with the numbers they brought to the battlefield.