What's going on?

Just why has David Cameron sacked one of his front-bench MPs? Is it because he suggested that being called a "black bastard" is all part of the ordinary cut and thrust of Army life, just as being called a fat bastard or ginger-haired bastard would be? Or is it because the MP - who was himself a serving soldier of distinction - suggested that incompetent black soldiers sometimes used racism to excuse their shortcomings. Either way, the MP is speaking from personal experience, so either Cameron is calling him a liar, or punishing him for telling the truth. Or is this just another case of a politician bowing to political correctness? Either way, I'm afraid he's rather gone down in my estimation.