When a user performs a double-enter on a sub bullet the indentation moves back one place.
This pattern creates a move fluid writing experience for someone that heavily uses bulleted lists and is already the norm in most word processing platforms (like Google Docs) or like in Dynalist ;).
I know that SHIFT+TAB accomplishes the same effect, but have a preference for the ENTER+ENTER since I already have to hit the enter key to start the next bullet.