I don't have one handy to check, but I believe the 2004 certificates had a previously unseen and fairly unequivocal restriction that stated "This certificate is not transferable" (or words to that effect.)

In previous years, the certs said something like void if sold or bartered, but in 2004 it seems they made the non-transferable, even as a gift. My suspicion is that this is in part why AP is eager to go to electronic certificates. That way, if they so decide, they can make absolutely certain that certificates are not transferred, even as gifts.