He didn't pass on the lock. At first she didn't wear it, but after figuring out how to make shelter when they needed to get away from the tuftcat (Stubtail's mother) she decided to wear it. Remember how Moonshade took issue with it? To her it was wrong that Ember wore the Chief's Lock, because Cutter was still alive (and still is). Eventually, though, Ember proved over and over again that she had indeed earned it, so that when the Shards War was over and the tribes met up Again Cutter told her never to undo her Chief's Lock. I'd even argue that by staying behind and hunting down Winnowill's creatures she earned it even more; after all she could have simply chosen to leave Djunsland behind and go "Let Winnowill's creatures hunt down the humans for all eternity. I don't care."