The bill of rights treats rights as existing in morality (whether based on natural law philosophy or religion) prior to and outside of written law. It does not purport to grant rights, but to prohibit the government from violating them.

In that framework, the US government is not to violate anyone's freedom of speech anywhere in the world. That doesn't seem to be what's at issue in this case though; the case was brought by a group of US-based WeChat users on grounds that the ban would restrict their freedom to communicate with people in China who are unable to use other communication services.