I suppose that Lincoln was addressing the American people in a manner which conveys that they (the electorate) are his boss. Its the same tone in other formal speeches, eg any State of the Union speech. Compare to examples of Presidents cussing.[1] These examples are usually directed at specific subordinates. Also a funny note to compare the example of when a celebrity cursed a former president, going against the hierarchy.[2] The celebrity had to apologize afterward. It seems to fit the theory well enough.

1. http://www.politico.com/click/stories/1006/great_swears_in_h...

2. http://www.metro.us/newyork/entertainment/the-word/2013/07/1...