The FBI had been working with the New Zealand authorities for a year before he was arrested. They wanted to arrest him on his birthday “because it would be a big party and all the assets would be there.”A Department of Justice official revealed this to ex-Senator Dodd, chief lobbyist for the Motion Picture Association of America. The FBI is a Justice Department agency.
The MPAA had wanted the US Congress to pass a law allowing for the overseas prosecution of people they considered were avoiding copyright. Pressure from a variety of sources killed the idea, and when Presidential candidate Obama went cold on the idea the MPAA threatened to scupper his fundraising in Hollywood.
So it looks like the fallback position was to keep DotCom in New Zealand for his birthday, confident that New Zealand government support, already demonstrated by neutering the industry unions, would return him to the US for prosecution. Alas for them, our Keystone cops and spies botched the birthday operation.
But that’s why he had to be kept in New Zealand. And that’s why there was political pressure from the National government to do so, once Simon Power had left the building.