I wonder if they only targeted people who had cheap airfare booked in December? Maybe they're targeting people that look like they're doing mileage runs? At least a few people here that got the offer (myself included) mentioned that we had plenty of airfare already on the books to hit the target.
In my case, my mileage run to Boston is costing me about $0.04 per mile which is a much better deal than what they're offering online. Of course, it would eliminate the need to make the run, but I'm looking forward to visiting with friends in Boston for a few hours anyway.