The grandparent comment asked whether a website owner would ever be unjustified in deciding who can use their website.
From a legal viewpoint, the answer is dependent on the complexity of state laws[1]. What a website owner can do with a website in one country obviously differs from what they could do in another country. Most countries have very weak anti-discrimination laws, and if they do exist, they typically only apply for very specific purposes such as employment discrimination based on age. These limited laws tend to be near impossible to enforce short of someone self-incriminating themselves. In some countries however, an example being Norway, laws against discrimination can be very strict and routinely enforced to the level of requiring all website owners to implement WCAG 2.0 at AA level[2].
From an ethical viewpoint, the Universal Declaration of Human Rights[3] states in Article 2:
"Everyone is entitled to all the rights and freedoms set forth in this Declaration, without distinction of any kind, such as race, colour, sex, language, religion, political or other opinion, national or social origin, property, birth or other status.
Furthermore, no distinction shall be made on the basis of the political, jurisdictional or international status of the country or territory to which a person belongs, whether it be independent, trust, non-self-governing or under any other limitation of sovereignty."
And numerous other articles are relevant, including Article 19:
"Everyone has the right to freedom of opinion and expression; this right includes freedom to hold opinions without interference and to seek, receive and impart information and ideas through any media and regardless of frontiers."
[1]
https://en.wikipedia.org/wiki/List_of_anti-discrimination_ac...[2] https://www.uutilsynet.no/english/about-us/903
[3] https://www.ohchr.org/en/human-rights/universal-declaration/...