@Bongo -
There is a fan-theory that says that the ponies did that for Luna to please Princess Celestia. After all, you don't want to displease royalty only to be thrown into a dungeon for your incompetence, right?

Though I, for one, don't believe the theory. I'm sure it's nothing more than a continuity error on the writer's part. Still, will be nice to see how they all feel about her.
Now, to find something to occupy myself for the next two weeks while I await for the newest episode. Where Twilight has NOTHING to send to Celestia on "The Magic of Friendship". Gee, I'm sure that will turn out well.
