The hypothesis I've heard is that Disney fears people will find the original Tron dated and campy. This might in turn make people less interested in Tron Legacy. It seems Disney will rather lose some DVD sales for the original than risk lower ticket sales for the sequel.
In that case you would think they'd tell YouTube to take down all of the clips from the original Tron.
The thing that makes me doubt this is that Tron Legacy isn't a reboot, it's a full blown sequel.
I don't recall any race specific issues in it - last I remember everyone was white, or a computer program.
And they did release it in the past - it isn't a "Song of the South".
You missed the point. Disney has always valued money over embarrassment in the past with regards to releasing movies, with the exception of Song of the South.