I don't care about the US perspective, I care about his perspective, and for the life of me I can't imagine someone so incredibly convinced of their own ability to hide their tracks that they'd be involved in a multi-year effort like that and think it's smart to go visit the United States. It's not like that would be the first time someone got arrested on entry. If it can happen to the CEO of a large multinational it can happen to Joe Random Hacker at least as easily. The interesting thing is they didn't arrest him on the spot, but they might well have.
He was running one of the most 'successful' illegal forums for what 7 years? When you get away with something for so long, especially in an 'industry' where you know every law enforcement in the world is after you, I can see why you would easily think you're invincible and never getting caught.
He was incredibly convinced of his own ability to hide his tracks because technically up to that point, his ability to hide his tracks was good. A self fulfilling prophecy of sorts.
Either that or he was spotted long ago, judged to be untouchable because he was still a minor and they let it go until they could charge him as an adult.
The indictment documents a pretty lengthy sting operation.