Four options why she does this:
1) She does it because she wants to have a foal of her own and Shining Armor can't or possibly won't give her one?
2) When the crystal subjects are not following the laws set by the Crystal Empire and it's ruler, she has a special prison to send them to other then a crystal dungeon. (a lot of elasticity with this option.)
3) She discovered it by accident when foalsitting Twilight Sparkle. (This storyline can go one of two ways.)
4) She does it for her own pleasure.
