The "problem" with unwritten rules is that people who never have been (and probably never will be) in situation in which you have to seriously decide how to take an attitude towards them (or generally how to act, sometimes maybe not even thinking about them) are first to judge. And preferring to go on without imagination and flexibility, they don't care about circumstances, the fact that every case is different. They chose the example which suits them, like 'Ullrich waited for Lance, Lance waited for Ullrich', add few quotes for better effect(Klöden, Vroomen)- and are perfectly happy with their superficial view.
You can't say that Lance knows his fairplay better just because he waited for Ullrich- he already had a secure position and huge advantage, not to mention that the accident was a bit more serious. And it's not like he was equally nice all the time, and like the stage during which Ullrich waited is the best example of this 'golden era'- if it really had been so perfect then, we wouldn't have had Armstrong not wanting to admit that Ullrich waited.
And of course, it's not like 'Andy did it first!' is a good way to justify Alberto's actions- but it is an argument when someone says that he didn't deserve to win. It's not like anything needs justifying though. All four riders (during stage 3 and 15) did what they thought was right in the heat of the moment- I find it really funny that someone blames them and even more when someone wonders 'was the race already on in both situations?'. You can't put a definite border, it's for them to decide (of course I mean when the case is controversial).
Thankfully, it's not like that's really a problem, as it only concerns people on forums and blogs, and all four riders seem okay with that, just like ex winners and pretty much everyone who knows what he's talking about from experience (Armstrong and Riis included).
What's interesting for me is (of course, purely hypothetically)- had the yellow jersey been someone else (and who could that be), would the others have waited? I mean, I can imagine others waiting in similar situation, but to be honest only under one condition- that the person in MJ, who has the mechanical, is the extemely charismatic one, with huge respect in the peleton. And Andy IMO isn't for many reasons (and I'm not saying I don't like him)- he's disproportionately arrogant, not the main candidate for winning, too nervous in such situations (who would want to lose time to anyone because of him fixing his chain for far too long?). Honestly, I would be totally surprised if Menchov or Sanchez cared the slightest about what's happening with his bike after he said that "they aren't rivals for him".
(And I wasn't also trying to say that riders would have waited for Contador, just wondering if there's someone for whom they would... I'd say that most probably for Cancellara, even though he won't ever be the main candidate for the triumph in GC.)