You are getting mixed up between the witness initiation ceremony and the shadowhunters initiation ceremony. They are night and day. Literally.
Oh yeah, probably. Maybe because after all, the early "Ninja" stake & luck of the top witnesses won't last forever if they are unable to correct what needs to be corrected, right?
And they won't have any chance to stop the climb to the top of the Shadowhunters' Jedis after their memorable and devastating Mortal Kombat finishing move.