I'm just genuinely puzzled as to why people write things like "The rest of this article is for Newcomb one-boxers only" when something like "The rest of this article concerns the subtleties of one-boxing, so if you don't care about that feel free to move on" would seem to be more accurate (and incidentally less inflammatory).

It's simple practicality. We don't want want boring rehashes of old arguments, that already have their own place. Even if I have general disagreement with some premises I take such a disclaimer and simply accept certain assumptions for the sake of the conversation.

Something along the lines of your suggested alternative may make this intent (at least, what I choose to act as if is the intent) more explicit.