Witches are a rather new phenomena. Their mutation, or whatever it is, has somehow also eluded the system. But the difference, is that whatever changed about them grants these people powerful, wild abilities. As I eluded a few times, a witch/warlock is what caused the collapse of District 26. It is hard to tell whether it was one alone, or many, that caused so much havoc. But that should be enough of a tell. These folks are a real threat to the current society. 'Magic' as it were, is seen as the reason the world was nearly destroyed in the first place. 

Yeah, nothing that crazy. If the ancestors could control time, they wouldn't be gone. Maybe artifacts might have some pretty crazy effects, but these are just relics. Not to say they are weak, by any stretch of the imagination.