The First Amendment and all that.

First Amendment doesn't apply to other countries, though. In the literal sense, that is - most OECD countries have some analog to the First Amendment in their own national basic laws. But the specific protections of our First Amendment do not restrict foreign governments from imposing criminal penalties on speech in circumstances that would not be permissible in the U.S.

You get into some pretty complicated jurisdictional reach questions when it comes to posts on a message board, of course. Generally speaking, Country X can't impose civil or criminal liability on a citizen of Country Y for acts committed entirely in Country Y. But that gets really messy when it comes to activity on the internet, which can cross international boundaries. When you post something on Twitter, are you publishing that content - but are you publishing that content in every nation where it might be read?

UK and Wales were notorious hotspots for libel tourism for a while, because their civil laws on speech did not previously differentiate between foreign and domestic speakers. So if a U.S. citizen said something libelous under UK law that ended up being published in the UK (among many other places), they could be sued in the UK even if they had little connection to the jurisdiction. That's since been amended to create a much higher standard for suits against people that aren't domiciled in the UK/Wales.

I'm not sure that would protect against criminal matters, though. And if you publish something in England that's a criminal offense in England, and they issue an arrest warrant in England, the English courts aren't going to recognize the First Amendment as being relevant.