Think about this: Catherine Middleton is not a princess in any shape or form, because she was born common. However, she will [edit: NOT] be if certain ramifications are met*. That's a whole different discussion, but because SA wasn't born into the royal family, just a high-class Canterlot citizen, he isn't a Wingacorn because he's only ceremoniously royal, much like Kate.*=EDIT: Entirely unrelated, but it's not possible for Kate to be a princess, ever. She may be Queen, but she'll have to make the jump from Duchess to Queen.