Kim Dotcom was running a bunch of servers in the US, which means he was running his business at least partially in the US. Secondly, arguably the majority of the infringing content was produced in the US. The US undeniably has significant interest in this case. Does someone become immune to the laws of another nation just because they cross a border? Tomorrow if a US citizen defrauds you, would you not want them brought to justice regardless of where they reside? International treaties exist for a reason.