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It's because someone somewhere links to that file, so they treat it as content as well as its true purpose.
The best example is in the search results you posted. #1 is Wikipedia expaining robots.txt. #2 is the White House robots.txt itself.
Look again at at the Wiki article and you'll see they link to the White House robots.txt
Why Google database show robots.txt file?
Brett Tabke experiments with writing a weblog in a text file usually read only by robots. Trenchant commentary on the world of search engine marketing.
www.webmasterworld.com/robots.txt - 2k - Cached - Similar pages
google's robots txt - [ Translate this page ]
User-agent: * Allow: /searchhistory/ Disallow: /news?output=xhtml& Allow: /news?output=xhtml Disallow: /search Disallow: /groups Disallow: /images Disallow: ...
www.google.com/robots.txt - 3k - Cached - Similar pages