I mean i know its to prevent the dragon slayers from becoming dragons but..does that really solve anything? i mean acnologia can turn from human to dragon at will so could they have not known that? They said it was also to prevent them from being like him but acnologia did those things on his own actions. Becoming a dragon had nothing to do with it if anything i would think not being able to turn into dragons would have the dragon slayers be at a disadvantage. I know they were just trying to protect natsu and the others but..