It's an example of a *qualifying statement*.

[This site][1] notes that, by far, "*I'm not a racist, but*" is the most common example people think of when they think of a "qualifier."

People who are [trained to spot liars][2] note that, when people lie, they preface their lie with a qualifying statement.


  [1]: http://racismschool.tumblr.com/post/19853619254/why-cant-you-just-answer-the-question
  [2]: http://old.iolaregister.com/Local%20News/Stories/Andnothingbutthetruth.html