I don't know, but I do seem to have encountered some of the most serious document checkers at DEN. The elite line was moving really slowly one day because I swear the security person was checking the BP name, the ID name, the ID picture, your face against the picture, the flight number, the flight time the flight date, your citizenship, etc. And I think they have always been serious about marking the BP in some way. So I guess whoever is in charge of security at DIA takes this all very seriously and decided that a stamp is a faster and more official method.