I feel like no one here remembers when Dmitry Sklyarov was arrested under similar circumstances. The US government has no obligation to seek out every potential arrestee no matter where they are in the world for every single crime that the US has laws for. But if the target of an investigation (whether they know it or not) sets foot in the US, then we shouldn't be surprised when they are arrested. And this is just another case with Def Con (so no, it's probably not moving out of the US, it didn't 15 years ago), I'm quite certain that these sorts of things happen frequently for other crimes of (relatively) low priority that are just outside our primary focus on this forum (technology).
And is the US any worse for this than other nations? Probably not. They just get more publicity when it happens. But every nation that has a legal system will do the same thing. If the Russians or the Brits or the Germans or the Swiss decide that Jtsummers is a suspect in a crime, and I visit and they realize it, I shouldn't be surprised to find myself arrested and barred from leaving the country.
[0] https://www.cnet.com/g00/news/russian-crypto-expert-arrested... - may not be the best article, it's the first one that came up on Google for me.