Wait...I got it right? My original reasoning that he could see that B and D were wearing white hats and the Psycho said 2 White, 2 Black...but then I read the part that said he can't turn his head to look at D behind him. What was the real reason, if I may ask?
It's because D didn't answer. If B and C had been wearing the same color hat, then D would have guessed his hat color. Since he didn't, C knows that he and B have different color hats. Therefore, C knows the color of his hat.
This might sound cruel but people commit suicide everyday. The media hits happen when somebody/site/etc uses the story to push an agenda and not real awareness. Social media is a real driving force if you can convince them to push something for a certain reason.
