Earlier quoted context omitted.
As others have noted, the US didn't just make up jurisdiction here or declare that they can prosecute anything anywhere -- the charges include an allegation that he transferred stolen BTC to a US-based company. That creates a crime the US has jurisdiction to prosecute. It really is time for people to stop being gobsmacked at the idea that once you get entangled with an entity in a particular country, anything you do…
I still find it bizarre though - they may never have left their bedroom and suddenly we have a country on another continent getting involved.
The idea that "on the internet" is a magical stateless realm has only ever been an idea in the minds of naïve geeks. It has never been reality.