Here, I'll be fair and explain my position on it.
The reason why someone can't be racist to someone who is a Muslim is because they choose to be a Muslim. Sure, you can discriminate against someone due to their faith, but it is based upon a choice, not something they can avoid. If someone called you Muslim because you're of an Arabic decent, that would be racist because they are assuming you are something based on your unavoidable appearance.
Tomorrow, however, I could say that I've converted to Islam. That would be a choice. I was not born a person of Islam and neither were you. It is a choice.