[url]If no mileage is credited to an account during the first nine months after opening [...] the Alaska Airlines Mileage Plan reserves the right to delete an account and reassign the number.
How is this possible? At least when you open an account from the website (dunno if there's another way to open it), you earn 500 bonus miles shortly thereafter. So how is it possible to open an account yet not earn any miles during the first nine months?