Edit: To the replies pointing out the protected class distinction: I'm well aware of it (as my example shows). What I am highlighting is that "freedom of association" isn't as absolute as the parent post implies, and that in the past the law was changed and that freedom was diminished when it allowed behavior we didn't like.
Anti-Semites are not a protected class. Political opinion is not protected in private space. The policies of these private platforms are clear and available for anyone to read, and include a ban on racist speech. If you want to stay on the platform, follow the rules.
You have no control over what color you were born, and you can't change it. That's why race is a protected class and opinions are not. Free speech only protects us from the government, not from private consequences.
In fact, if you were to start a group that wanted to destroy white Europeans (like how the Klan wants to destroy non whites, Catholics, Mormons, Jews, and sanity itself) then you would quickly find yourself ostracized.
I would bet an anti white group would quickly find itself branded as a terrorist organization in the US.