This doesn't make the fundamental problem here go away: they arrested someone who was not in the United States when allegedly violated a law of the USA. The USA does not have jurisdiction over the entire world. If they thought they had a case, the right way to do this is to issue an arrest warrant and ask the UK to arrest and extradite him. If this was a thing, international travel would halt because you would need to countercheck everything you've ever done online with the laws of the country you are flying to.

Consider this fictionary tale: you fly to Budapest for a fun trip. You are jailed for posting the Soviet hammer and sickle on your Facebook two years ago -- it's a crime under Hungarian law to use that symbol. Do you think this is right?

Here's the law:

Any person who: a) distributes, b) uses before the public at large, or c) publicly exhibits, the swastika, the insignia of the SS, the arrow cross, the sickle and hammer, the five-pointed red star or any symbol depicting the above so as to breach public peace - especially in a way to offend the dignity of victims of totalitarian regimes and their right to sanctity - is guilty of a misdemeanor punishable by custodial arrest, insofar as the did not result in a more serious criminal offense.