Even if the tattoo was a joke, the idea of "play stupid games, win stupid prizes" seems to apply.
Except for the $10mil lawsuit filed by the grieving family with evidence of the images all over friendface stating it was a joke. I agree with the Darwinistic aspect if it was a joke, but I understand why a hospital wouldn't want to take the chance in a litigious society where you can get millions for tripping on a watermelon pallet.
It IS a very good point about "How many carry DNR paperwork?" My paperwork is pretty much "just let me die", but how would a hospital know that if I were hit by a bus? Eventually they would find out, but not until they potentially put me through a bunch of painful "lifesaving" measures that had me looking like a Borg.
Edit: BSL and Dogmush touched on my points as a typed.