Oubliette - "Because it's a relatively innocent 'guilty pleasure' and you can't get DF'd for it."
When I'm in a cynical mood, that's the explanation I'm most inclined to favor.
JustVisiting - "Another sister that attended dressed as Princess Leia was later counseled by the elders for 'idolatry'."
Even money says they wouldn't have counseled her if she'd dressed up as a Bible character.