I'm finding this discussion a little frustrating because there is very little mention of the fact that we are talking about
tournament rules. And that is crucial here. It is only because everyone entered in the tournament has equity in the prize pool that soft play is an issue. (Some forms of collusion are obviously an issue in cash games, but soft play and chip dumping are primarily tournament issues.)
As such, I think DCJ001's sports hypotheticals have little, if any, applicability as to why soft play is against the rules. It's not because spectators don't think the players are individually being competitive enough or aren't taking the game seriously enough. That has nothing to do with it. It's because the person who soft plays gives an unfair advantage in the tournament as a whole to the beneficiary of that soft play, which is to the detriment of the other players in the field.
(And sometimes a less cutthroat mentality in one-on-one competitions can actually be considered great sportsmanship, see e.g.,
http://www.cbsnews.com/stories/2008/05/01/earlyshow/main4061276.shtml)
Likewise, Nick C's point about not wanting to take a little old lady's last $20 is fine for a cash game. I can respect that. Let her keep her $20 forever and ever and it really doesn't impact anyone else. Besides, even if she lost it, she could reach into her purse and be in the exact same position that she was before. But if she keeps her T20 chips (which she can't just buy back) for longer than she deserves to, everything changes. She might win much more in tournament prize money -- money that a different player would have won had the little old lady been eliminated earlier. (The Vindy.com article mentioned the tournament issue, but extrapolated to conclude the any soft play in any poker game is cheating. And I do not agree with that.)
Also, to clarify what I think Matt Savage and others have said about the rules -- (1) there is no TDA rule that
specifically states that a player holding the nuts when last to act on the river must bet/raise, but (2) there is a TDA rule that applies to that situation, namely the rule that prohibits soft play (Rule 40). Like many of the TDA rules, the tournament director does have to interpret the rule and apply it on a case-by-case basis based on the specific circumstances involved. But I would agree with the position that in most cases, checking the nuts should be considered soft play in violation of Rule 40 (more specifically, I agree with Mike's earlier post that broke it down into exclusive/non-exclusive nuts, etc.).
I can think of a few situations when an argument could be made for not giving a penalty:
(1) In a Heads-Up Tournament or when down to the last two players in any tournament. No one else's tournament equity would be affected, so players can soft play all they want here -- even if that lack of competition might sicken DCJ001. So if Eric Bana wants to muck the best hand against Robert Duvall when they are the last two left in the championship event, that's his prerogative.
(2) An honest mistake where the player didn't know they had the nuts -- especially if it's an inexperienced player new to Omaha or something where it would be understandable that they might not realize they held the nuts. This would have to be judged on the totality of the circumstances, and warnings would have to be given.
(3) This one might be a stretch, but when approaching the bubble in a large tournament, because it can be to the advantage of the big stacks to stay in bubble-mode for as long as possible so that they can chip up by stealing from the small to medium stacks, if one of the big stacks were to soft play a short stack to avoid the bubble bursting, an argument could be made that it was not really soft play for the benefit of the short stack but a strategic move by the big stack for their own competitive advantage. The soft play would still negatively affect the other players' equity in the tournament, so I'm not sure about this one, but I think it's at least an interesting argument.
I should say that I am not and have never been a working TD. I'm just an amateur poker player who takes a more-than-normal interest in the rules. And I've only been playing poker at a recreational level for a couple of years. So my two cents are probably worth just that.