I have not been accepted yet (or offered an interview for that matter, oh the sorrow) but I will play devil's advocate for a second.
Let's say you interview at several schools and are wiating for the final decisions. Your 1st choice school send the the big "YES" and you accept. You have yet to hear from the other schools, but you are curious if they will accept you. So, you choose to not inform them right away that you plan on attending another school. One has a right to know if other schools would have accepted them, especially since the applicant worked so hard (interview, GPA, applicaition process ect.). There is a certain peace of mind knowing which schools officially accepted you and which ones didn't.
In the end, the waitlists will move.