Actually, promo codes do have restrictions, and this is indicated on the sign-up screens. Note Andrew already indicated this in a previous post on in this thread.
Not all promo codes are available in all regions, and some regions may not use promo codes at all. If a promo code entry box is not displayed, than a promo code may not be used (and it will not be displayed for all visitors).
Questions about a given visitor's experience with this should be directed to support in a support request.