Letterman has for 7 years made fun of the president almost nightly. Not just when something funny hits the news, he goes out of his way to show clips of Bush stuttering on a word or misstating something. It's not humor, it's ridicule and on a regular basis. Even the least intelligent get the point after awhile.
To keep repeating the same old canards about the president undercuts the image some have of the man and the office and to do it intentionally five nights a week is sabotage.