I think that their statement makes them not have to. My interpretation is that, if you haven't received an award notification yet, you're on a waiting list. As people reject the award in preference to others, more awards go out. Once all of the awards are accepted, the rest of the waiting list will get a rejection letter. The difference between this and getting wait listed for a school is that a much smaller fraction will reject the award. So, yes, it'd have been nice for them to send out a "you're on the waiting list", but I think that they would view this as a waste of time.