To put it mildly, except for matters of warfare, mayhem and destruction, ISIS has never been known for thinking in a logical or linear fashion. They did not kill these men. They only disabled or severely maimed them in a horrible way. So how does this help their cause, when the men will still need to be fed, sheltered and transported.
Taking all human compassion out of consideration, this tongue severing creates an ongoing expense. Perhaps they assume the men will eventually commit suicide to escape their hell on earth. So the Mullahs have the short term sadistic satisfaction of torturing one of their own, as a tool of intimidation.
They are evil. Most join because it’s a way to kill and rape and maim without restraint, not because of any belief in Islam. They kill for pleasure. And they kill each other given half a chance. Don’t try to put logic to it. Evil has no logic except destruction including destruction of self.