Valve also does business outside the US. American law doesn’t clear them of their legal obligations in other countries. And besides, legality and morality are not always the same. Providing a platform for hate speech is supporting hate speech, and as far as I’m concerned that’s unethical regardless of whether or not it’s legal.
Oh for the love of fuck. Americans. Your country isn’t the centre of the universe. It doesn’t matter where a company’s headquarters are. If you are doing business in, for example, Germany, you have to abide by German laws. Being American isn’t an excuse and it doesn’t shield you from consequences for breaking the law.
Also, the big issue is that Valve isn’t actually using their right to refuse service. People can spread all sorts of bigotry via Steam’s discussions and groups without Valve acting on it. They’re providing a platform for hate speech and that is inherently immoral, regardless of what the law says.