Rights guaranteed by the US Constitution—with a few exceptions, such as the right to vote—have generally been held to apply to everyone under American jurisdiction, not just to citizens. Though Assange is not a US citizen and lives outside the United States, First Amendment protections would in theory apply if the US government attempted to prosecute him under its laws, e.g. following an extradition request.
Rights guaranteed by the US Constitution—with a few exceptions, such as the right to vote—have generally been held to apply to everyone under American jurisdiction, not just to citizens. Though Assange is not a US citizen and lives outside the United States, First Amendment protections would in theory apply if the US government attempted to prosecute him under its laws, e.g. following an extradition request.