And what if you are hospitalized by another hospital? Seems like that's what happened to this guy. If he didn't have that tattoo they wouldn't have even bothered to figure out if he had a valid DNR document before resuscitating him.
Tattoos are really great signalling for motorocycle gangs and hipsters, but the nuances that one can use to discern satire, hubris, or seriousness just don't work well in an emergency room.