The rules specifically indicate that soft play is not allowed.
If you hold the nuts and are last to act, and fail to take an aggressive action, it is likely that either:
A) you are taking it easy on your opponent and are content not to try to milk him for more chips
Or
B) you do not know that you hold the nuts.
Allowing the action to occur for reason A is "soft play" by any definition. I assume you agree that this is unacceptable in tournament play. B, well, is just probably a product of ignorance or stupidity.
You claim to have only seen elderly players do this, who fall under B. FWIW, I have seen players take it easy on each other for reason A, just to "keep things friendly". Not necessarily because they know each other, but because they just felt sorry for their opponent (the opponent has been whining about getting bad cards, the opponent is attractive, the opponent comes from the same home town, etc.) I have also seen players 'misplaying' the nuts on the bubble - where betting the nuts may have meant a short stack (not in the hand) cashing. Inevitably, the short stack cries 'soft play! Collusion!', while the player with the nuts claims he didn't know he had it. And sometimes, it really is a question of whether the two are strangers or whether they know each other, Sometimes, the two players in the hand are simply from the same country... And that alone makes people suspicious.
My point is,,, we cannot prove anything. As a third party observer, we can never truly know whether the situation is A or B. We are not mind readers. So we have to draw the line somewhere as a default. I take your point that the default position might just as well be to assume that B is true, and place the burden in the TDs hands to prove the situation is really A to justify a penalty. But, for better or for worse, the well-accepted custom is to assume that A is true ("if it walks like a duck, and quacks like a duck...") and have the player convince you that in fact B is true to avoid a penalty (assuming the TD is even willing to give the player the benefit of the doubt).
I respect your position that perhaps it should be the other way around, and maybe that is something worth fighting for. But I expect it will be difficult to convince the majority that this ONLY happens because players are unaware of their hand. While I would like to believe that checking the nuts is always an honest mistake, I just don't believe that is true. I'd rather give your clueless elderly players a warning and penalize the players who ought to know better. Most TDs will argue that we need to consider not only allowing players to make mistakes in misreading their hands, but also their obligation to protect other players in the tournament. Frankly, the more persuasive argument to me that is sometimes raised is that the person with the nuts is effectively being penalized twice: once from the penalty for soft play, and once for winning a smaller pot that he may otherwise be entitled to.
As for being in the rules, I don't think it is fair to say that it is not in the rules. Could this situation be presented explicitly as an example? Maybe. But as Chet suggests, it may be unrealistic to enumerate each and every possibility of what constitutes collusion, what constitutes soft play, what constitutes unsportsmanlike conduct, etc. within the rules. I don't see anything wrong with grouping actions into categories rather than having lists of specific examples in the rule book. But if people feel strongly about it, we could always try to come up with a non-exhaustive list to aid in interpretation, although I am also ok with the way it is.