Gone are the days when an appearance on late night television actually meant a boost in sales. In it's heyday of the 70's, an appearance on Saturday Night Live meant a sales bump of 100k or more the next week, but those days are long gone.
If U2 really wanted to goose their sales via television they'd play daytime television like Oprah or Ellen. If they wanted to be hip they might have chosen Jimmy Kimmel or even Conan, but Letterman is no longer considered hip and certainly won't help sales as a result. Hard to believe that's what it's come to these days, but the major influencers are not who they once were. Then again, perhaps U2 could care less about sales at this point in their careers and just want to party with Dave.