Don't play daft. Mega was operating a business aimed squarely at the US market and built off the intellectual property of American companies. The US interest in seeing that case prosecuted is clear.

If NZ wants to alienate one of the world's largest markets, their largest military ally (and the guarantor of their security, particularly in their shipping lanes) to stick up for a foreign national running a criminal copyright conspiracy on their soil, they're free to do so. Don't get all huffy when they decide not to let that happen.