Actually BI cited other sources, namely Amazon's response and further email evidence from the lawsuit, which was somehow omitted from Kivin's document. In it the Amazon SVP statement notes that the campaign was set up to pay $10 as a reward to Discover card users, not tied to 1-click conversions. He also notes this was reviewed with Discover mid-promotion and they elected to continue within budget.

There is of course some promotional benefit to that, and it's not unusual in the card industry. While some card benefits are conditionally offered to new signups (e.g. promo APR), most are offered to the entire class (e.g. cashback rewards).