Then why is it you always seem to want to "Redefine" the topic when it is plenty specific and their is no ambiguity?
Honestly your post take on the flavor of the "balloonist who is lost."
It only seems to be YOU that sees the subject as redefined. If you are speaking of subset analysis in original posts, the superset is always part of the subject, therefore part of the conversation, much the same as if a superset is the subject of the original post, subset spinoffs are part of the conversation.
Three examples. On the Fox News survey, it is a subset analysis by Neilsen. The superset information is very germane to the conversation for perspective. Same with the Today Show posts, which when their ratings [and that of NBC News] were brought up is a logical expansion of the conversation.
Then, on Milfy's Hispanic foreclosure thread, two natural progressions of the conversation were to look at the economic breakdown of the Hispanic population in the area and to look at the income impact of the real estate profession. Those were three subsets of the superset of the subject and all natural progressions to the discussion.
And in all three of those examples, the original posts, either through the poster or through the accompanied linked article contained reasonable ambiguity and were not specific enough. This led, on all examples to additional questions and information important to the continuation of the conversation. That is NOT "redefining" under any definition of the term.