I think this is true, in some cases. I have heard of people being "waitlisted into oblivion." In other cases, not true. I am a waitlist at Maryland, and I confirmed with them a few times that I would receive notice if rejected.(Although they were not specific about the timing.)
Although I'm with you in that schools should notify, to be fair they are sometimes placed in a difficult situation when someone withdraws at the last minute (or even after orientation). In such cases, they would need a list of people to call on. An extensive list at that, because the chances that someone would duly oblige to such a call that late in the game would definitely be remote.