Better safe than sorry, IMO. There's people (like me) who wishes that they can unwatch the teaser, and as such am glad TWH/Sprocket is addressing the issue (though IMO a second version is in order, even if all whats changed was the title to indicate there's spoilers, because theres a lot of people dls the torrent directly and never see the scarywater page, ex. tokyotosho).
As for the no outcry in Japan, you just said the root of it all. The disk was sold out; in other words, its really popular. Otakus being otakus, what percentage of the people who bought the disk do you think have played the game before? Chances are, its close to 100%. As such, them seeing the teaser spoils nothing at all!
As for the analogy to Kanon/Air...
Spoiler for for Kanon end and teaser of Clannad only, nothing more, nothing less, I promise:
it's a far cry from those. Taking Kanon for example, in the commercial, sure they did show the tree cut and Yuuichi shouting (or for all the uninitiated knows, he just looks like someone gasping for air). But how would someone with no knowledge of the game/previous series know that Yuuichi was looking for a girl, let alone know that the girl is Ayu, and going further, know that she's in a coma with her projected self running around town? It's a VERY BIG stretch to get from point A to point B, and as such, doesn't really count as a spoiler.
But the clannad teaser? I intentionally lied that one has to connect the dots to get spoilered; you don't even have to get from point A to point B, as point A is ALREADY a spoiler. Right off the bat we're told that it's Tomoya and Nagisa we're talking about (how many times did Tomoya and Nagisa's name mentioned?) , and that she 'disappears forever' in the end. Everything there points to her dying.
And even in the best case scenario that Nagisa is all well and kicking in the end of the movie OR that its really some other girl who died/disappeared, the fact is is that people who have seen the teaser already knows that there IS going to be a disapperance/dying scene, and a major one at that. Knowing that there is a scene like that alone would take away from the emotional impact, as the shock factor is gone.
As such, because even in the best case scenario one has the major potential loses enjoyment of the show because of knowing things from it beforehand, the teaser can only be classified as a Class A spoilerific item. In particular, it's the very script of the teaser thats spoilerfic; the images show diddly squat whether you understood what was saying or not.
As for your avvy, its not in the prelude (I just watched it). Heck, even if it was, showing it would probably have it flow over people's head, because some people would be misdirected that she's an angel sent by heaven (when she really is just a girl in a coma), while most would just think its some silly decoration thats not worth mentioning at all. It wouldn't spoil a scene at all.
And for the record, I'm not mad at all with Clannad being spoiled some for me, as I'm not the kind of person who cries over spilled milk. It's just I'm just a tad bit miffed when people try to underscore the spoiler factor of some things, because this might convince people that its actually alright to see it thinking they're safe. I still stand by what I said before...
Avoid the teaser like the plague if one doesn't want to get spoiled!