User blog comment:IamJakuhoRaikoben/Chapter 413: The Book of E.N.D, Review./@comment-24499036-20141222202543/@comment-9595671-20141223052935

Remember how Zirconis said Acnologia turned into a dragon? He said it was cause he kept killing them and bathing in their blood, Zeref might have had a hand in his current state but he might have not been the one who actually made him a dragon himself.

My best guess is that being inside them made the Slayer's magic "think" they were already dragons, that way even if they slayed one they wouldn't start changing. When you remember the war it was the pro co-existence dragons who taught humans magic to fight the pro devouring dragons, then the humans starting killing the ones who wanted to live with them.

It seems like a "power corrupts" setting and they just wanted to avoid having their children become monsters they would have to put down.