Why is Disney's "Song of the South" movie considered racist ?

I understand that this movie is an American classic, and for certain reasons it's unavailable for purchase in the U.S.
Why?
Was there ever an edited, or more PC version available for viewing?
Update: I have never seen the movie, but it's featured song "Zip-A-Dee-Doo-Daa" is a classic Disney song, and I understand that the movie was also graced with several Academy Awards, so how come it's shelved for poor taste ?
Update 2: kokopelli - it's out-of-print, unless you know otherwise please post a link, but spare us from a cheap bootleg version for sale somewhere.

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