It's not like they don't know they're on camera, and on top of that, Van Etten was up a game already. Who would intentionally cheat in a situation where you're pretty much guaranteed to get caught, and you don't even need the advantage that badly anyway? The only thing I can think of is that Van Etten told the judges "Yeah, I realized it a couple of turns later but didn't say anything."

By the way, speaking as someone who's played my share of paper Magic and made more than my share of judge calls: call the judges when this happens. Their top priority is to fix the game state, not to punish you. Sometimes if the game has progressed too far to fix, they'll let the mistake stand. I don't know offhand how enforcement differs at a high-level event like this, but I think there's a real chance that Van Etten could have salvaged a match win out of this if he'd called the judges on himself in time.