Seriously folks, please know what you are talking about.
Businesses are not required to honor any coupon/advertisement that is a mistake, this is the actual law. However, a business may choose to honor a 'mistake' so they do not offend their potential customers.
Also, I do not see how Best Buy could 'deactivate' any of these gift cards because no fraud was perpitrated. The terms of the coupon were there and clearly did not limit the number of coupon uses or limit giftcards, the link was an open link on the website so no hacking is required, it was a printable coupon not a special hard copy. I'm not sure what basis they would have to reclaim their goods/gift cards (it wasn't theft). Best Buy sold you a product at a price they offered.
They might have chosen not to honor the coupon, but once honored and no fraud I don't see how they can go back. Gift card sold for money equals a completed sale/deal. Ever hear of buyer beware, same thing here except seller beware.