Why is that? Is private business obligated to follow host country's foreign policy? My instinct is to hold the business responsible for these decisions.

Yes, a private business is obligated to follow the laws of any country where they hold assets that can be seized by the police of that country.

Censorship is a law in US now? Or, are you saying that it's law to follow its foreign policy? In that case you sure have a round-about way of saying that.

The Trading with the Enemy Act empowers the Office of Foreign Asset Control to come down very hard (million-dollar fines and years of jail time) on anyone dealing with a sanctioned nation (N.Korea, Iran, Cuba) or anyone on the list of Specially Designated Persons and Nationals. This is absolutely the law, yes.

The accounts were closed because they were business accounts from sanctioned countries? I didn't realise that. I had misunderstood that Github was enforcing the said countries' censorship.

Sometimes, yes. If the host's country's government applies sanctions to your country, they may have to stop allowing you to use their services.