David Letterman, as most know, plowed a bunch of women of various ages who worked for his show or for CBS. John Ensign had an affair with the wife of one of his staffers. Neither is particularly admirable. Each is bad in it's own way. Ensign is a politician, which means he's held to a higher standard than regular people. He also betrayed his wife and a close friend. Letterman is a prominent television host, who makes a living off joking about foibles similar to the one he's been found committing. He also likely traded on his stature to gain the affection of the young women.
However, to the NYT, one is possibly criminal for attempting to assist a departing staffer (albeit the cuckolded husband) in finding a job back in his home state, a common practice for Congressmen and Senators to engage in with their staff. The other is a civilian hero for turning in the would-be extortionist, who was the boyfriend of one of Letterman's flings. One gets judged, the other gets praised. One is conservative, one is liberal. It's easy to match the ideology in the latter sentence with the treatment in the former.