I first read about it in another thread on these boards. I checked the same Web site and also found nothing, but there's an e-mail link to contact them with questions. I used it and got the reply mentioned above.

I can only assume that they don't want to offend non-US travelers by making it too obvious. Or perhaps they don't want too many people to use it - but that sort of misses the point of a promo, doesn't it?