User blog comment:FairyRaven/Fairy Tail Chapter 375: The Supernatural, Review and Commentary/@comment-7418318-20140317103924/@comment-5840892-20140317164801

Actually to me this sounds like it could truly be legit.

They say that the curses they have are stronger than any magic. (going of stream's iffy translation here.) And if the curses draw power from something other than magic, and truly are as strong as the Demons claim, it makes sense that there would be a drawback of sorts. I have a way that type of thing could be explained without it sounding completely stupid (although to me personally, it sound stupid at all.):


 * The curses are a a Double-Edged Sword. They have a great power (or claimed to have great power so far) so it must have a draw back of sorts. Since the curses are claimed to not draw power from magic energy, there is the chance that some of the curses power comes from the "Books" that Demons are from. And that by being a certain distance from their Book the Demons lose some of their power.

The alternative is that the Demons were holding back before because they underestimated Human Mages, or something like that.