Earlier quoted context omitted.
Non-Americans are certainly protected by the Constitution while in the US, but I thought there was still a legal debate about whether the US Constitution fully applies to foreign nationals while outside the US. Hasn't that long been the legal excuse used for things like international mass surveillance and what goes on in Guantanamo Bay?
The First Amendment doesn't apply to anyone while outside the US. A US citizen in Saudi Arabia doesn't have First Amendment protections there. For the US to prosecute Assange, they have to prosecute him in the US, where he does enjoy First Amendment protections, as the First Amendment limits what Congress (and via the Fourteenth, state/local as well) can do .
There seems to be a precedent that US citizens have different legal protections from the US government while abroad when compared with non-citizens. That is why US citizens involved with Al-Qaeda, ISIS, or similar hostile organizations will occasionally receive a domestic trial in the criminal justice system while non-citizens generally only face military trials if anything.