"Half-Kenyan" wasn't the insult, it was "ancestral dislike of the British empire" - the full quote was:
"No one was sure whether the President had himself been involved in the decision. Some said it was a snub to Britain. Some said it was a symbol of the part-Kenyan President's ancestral dislike of the British empire - of which Churchill had been such a fervent defender."
The decision in question was the removal of a bust of Churchill from the Oval Office. Obama's response is here:
http://www.bbc.co.uk/news/uk-politics-36112694