It's an example of a *[qualifying statement][1]*.

[This site][2] notes that, by far, "*I'm not a racist, but*" is the most common example people think of when they think of a *[qualifier][3]*.


  [1]: http://www.thecriticalreader.com/blog/item/272-qualifying-statements.html
  [2]: http://racismschool.tumblr.com/post/19853619254/why-cant-you-just-answer-the-question
  [3]: http://ahdictionary.com/word/search.html?q=qualifier