I can get the "there were US customers" - but why not Europe? Or Japan? Or Russia? Or Australia? I'm sure BTC-e had customers from all over the world (and money laundry is pretty much a crime everywhere).So, when does it become "you have broken the US law and you are under arrest"?. Does it work the other way around too? If you start a gay social network in US, can Russia come in (the first time you are flying in one of the Russia's partners territories) and say "you are breaking Russian gay laws, you are under arrest"?
The best way to think about this is in terms of the laws of the country you are in.
We might be a outraged that Otto Warmbier was treated so harshly (killed really) by North Korea for allegedly stealing a propaganda poster, but at the end of the day we understand that he went to North Korea and in North Korea, North Korean laws apply.
The next step is to ask about actions that took place outside the country you are in but are illegal under that countries laws. So if, for example, Seth Rogen who famously made a movie making fun of Kim Jung-Un were to travel to North Korea we would not at all be surprised if we were arrested and sentenced to a very harsh punishment.
Finally, consider that extradition laws are just like any other domestic laws. The treaty between Greece and the United States means that for the purpose of a lot of criminal laws, physically entering Greece is the same as physically entering the United States.
In this case it was ultimately Greece that decided that what Vinnik did was sufficient to take away his freedom. He subjected himself Greece's judgment by going to Greece. If he had stayed in Russia he probably would not have been extradited.
Going to another country is something many of us take lightly, but it can be a really big deal.