I think you're missing the primary principle of discrimination law... the reason for why your boss "likes" you and rewards you makes all the difference in the world in the eyes of the law. If your boss likes you because your a good worker, or a top performer, or have a great personality, etc. that's one thing. But if they "like" you or a certain group of employees, because of things like skin color, gender, ethnic background, or willingness to submit to their advances, and steer extra goodies towards those they "like" at the expense of those that don't meet that criteria... then that's an entirely different matter legally.
Again, I'm not saying that Letterman is guilty of such discrimination towards young female workers on his staff, but there's been enough information stated that makes it a real possibility and I have no doubt that WWP's HR staff has been working a lot of extra hours and consuming above average amounts of Rolaids as this stuff has surfaced in the last week.