Ryan - I'm with you on this one.
IMHO, if you used screws that passed through the interior of the box, they would have to have their threads covered, and be within 1/4" of the ends or back of the box.
The screws in pic. 3 are not paasing through the interior of the box. Since they are not passing through the interior, they do not need to be within 1/4" of the ends or back. Since the threads are not exposed, they do not need to be shielded.
The only other way to mount non bracket-equipped boxes would be to mount them to a board tacked between the studs by screws driven through the back of the box.
