That's not confusing, what's confusing is his lack of direct association with the USA. There are a handful of other countries that would appear to be more responsibility for his actions.

Given that Greece is willing to extradite him and we haven't heard anything from the numerous countries where this guy has a business presence, I suppose most of the other countries involved are happy to have the USA deal with this guy.

Not every country has the resources to track down billions of dollars' worth of BTC transactions and use them to build a bulletproof case against a specific person. So if someone else offers to do it for free, why say no?

He conducted transactions with US persons as part of the illegal conduct. That's a sufficient nexus.

Ah, but the fact that there are a handful of other countries doesn't change anything - he can be prosecuted in all and each of these countries for various parts of that scheme.

USA prosecuting him doesn't mean that other countries can't, it simply means that USA was a bit quicker to do so. For example, it's technically possible that he's acquitted in USA for the AML part, but immediately after that gets extradited to Japan for some charges regarding the MtGox hack.