"must .... have provided Blizzard with a valid credit card which can be used by you to purchase World of Warcraft subscription game time on a recurring basis to participate in this offer.A one-year subscription can be paid in two installments of six months each. A reasonable person, such as myself or Wilhelm might reasonably have assumed paying for the second installment back in April insured that our commitments were met. Blizzard has chosen to interpret this commitment to mean that you cannot withdraw consent for recurring billing until the expiration of your pass term. The account page will not allow you to cancel recurring billing BEYOND the duration of the annual pass until AFTER it has expired. Customer service also will not help - the best advice Wilhelm could get was to attach a pre-paid credit card with $1 on it so that it would pass the "is this a credit card?" check but fail to bill.
...agree to maintain a fully paid up, World of Warcraft license that is in good standing with Blizzard Entertainment, Inc., or its authorized licensees for a twelve (12) month period beginning on the day that you sign up for the offer (the “Twelve Month Commitment”)."
Personally, I have a bit over a day between the time the system claims my commitment will be satisfied and the time of my next billing. Wilhelm was somehow less fortunate, and has only a few hours - according to his commenters, customer service is claiming that they will refund any charges resulting from being unable to cancel between the end of the annual pass and the next bill date.
My best advice would be for annual pass customers to check their billing information and, if they intend to cancel for whatever reason, to change their billing increment down to one month to ensure that if there is any billing dispute, the amount at issue is as small as possible.