The 1st Amendment is a moot point as Assange is not being sought for "speech" he has made but for criminal conspiracy and providing material support in furtherance of a crime. The US Constitution does apply to anyone the US Federal Government interacts in my opinion, as it is a definition and limitation of the powers of the US Federal Government. I know it hasn't been applied in practice many times and there are Americans who disagree with this stance but they're wrong just like some people were wrong about slavery.