Gee, even if Letterman is evil enough to joke about the sexual assault of a 14 year old girl, you'd think that he wouldn't do it because he was afraid that people who were offended would go to his online Ticket Request Page
http://lateshow.cbs.com/latenight/lateshow/show_info/tickets/form/ and enter the lottery for tickets to his show, just so that they could say, "Uh, no thanks." when Dave's folks called to say they had won. In fact, if a person was really made at ole Dave, they could accept the tickets, then not show up.
But, I'm sure Dave knows show business better than me. That is why he is rich and famous.