When personal data is easily swapped and shared on the network, a couple of keystrokes gets you, say, a United reservation, frequent-flier number included, plus the kind of rental car you like and your hotel reservation--even listings of the movies or restaurants that have opened since your last visit.
Evidently Delta is not worried about it because not only is the number printed but also the record locator number for the reservation and of course the name of the passenger.