User blog comment:IamJakuhoRaikoben/Chapter 372: Breach, Review./@comment-4489126-20140221150500/@comment-7418318-20140221151011

While I like the idea of them being partly sealed, if that'll turn out to be the case I'm hoping for a very good explenation as to why such a seal was put in place.

Maybe something like the reason for the seal was to make sure they couldn't possibly disobey E.N.D. or Zeref and the weakening just being a side-effect.