They will make money from this promotion. For most of the hits, the DM partners are buying miles from US so US is increasing its cash position, cash flow, etc. Of course, the miles will go on their books as a liability but I won't be surprised if US increases the miles for redemption. Even if they don't increase the miles needed for award travel, they can control the reward travel seats. An article in the USA Today this year reported that US has the lowest redemption\reward seats ratios among the legacy airlines.