There is - of course - rules around this. Certain qualities like race and sex are protected classes against which you cannot discriminate. To your example - a nightclub very well might discriminate against someone with blue hair.

Political speech is not a protected class.

This is correct.

I have to ask, what is the point you are trying to argue? That it shouldn't ever be possible to enforce rules on internet communities?

You should not have rules for behavior outside your community. Second, your rules should be restricted to actions that have an effect on your community.