He doesn't know this because he is Chinese. This kind of racism is a very small part of Chinese culture, because there are so few Black people in China. I've seen the Lawrence Olivier Othello (I saw it in English class in fact, when it first came out) and the character is noble brave and intelligent. The only reason that it is offensive is that it reminds people of other portrayals which are offensive. Prof. Sheng, being Chinese, was unaware of that history.

There are lots of things you or I would do that would be offensive in China. I'm sure Prof. Sheng would be familiar with those, and I hope he would forgive us if we made similar errors.