Board Thread:Manga/@comment-5774380-20161007195718/@comment-27321453-20161011065947

Tausendberg wrote:

Maybe the dumb dumbs are actually the result of the Marley's experiments into trying to make more shifters. Yes that's a possibilty and when they kept failing, they opted to use it on any Eldian revolutionary person and send them to the island seguing their own people.

So essentially they squeezed a benefit out of their failed experiments.