I think you're confused, here is the robots.txt from this site (linked by you)

User-Agent: * Disallow: /x? Disallow: /vote? Disallow: /reply?

user is not disallowed anywhere, vote? and reply? and x? are, but they are not content, they are actions, basically all of the content of this site is indexed.

Your idea of site owners being selective about SE makes no sense to me. Would users then have to use 5 or 6 different search engines to accommodate all possible webmaster preferences?