I'm guessing that DL figures that they would have to spend that much to fight a union anyways, so may as well fight the unionization efforts this way.
In other words, well I think it's great, I'm not totally convinced that DL is somehow uniquely benevolent among airlines. Rather, I think they find themselves in a unique economic and labor dynamic in which they are both very successful and also the only legacy airline to not be fully unionized, and are acting in the best interest of the company.