I feel bad for the hunter, as he seems like an honest guy who didn't understand the importance of immediately tagging your elk, but his logic that the animal doesn't need to be tagged until you are transporting it doesn't hold water.
By the time you have your critter in the truck and are transporting it you may very well be headed to your garage to unload it. Poachers are most vulnerable in the field, before they have got their critter in the truck (under a tarp, under a topper, etc) If the minute you approach your animal you cut your tag, there is not doubt or chance that you're up to no good.
Before the hero shots, the gutting, and most definitely the pack out, tag the animal.