The constitution only protects you from government censorship and even that has limits. The constitution allows organisations to make their own rules and regulations about what can and can't be said.
Secondly, BZP is not a monarchy, democracy, oligarchical, autocracy, dictatorship, or technocracy. It's not a nation, a kingdom, a country, or a city state.
It's a website; It has rules, not laws. It has a staff, not a government.