Because their copyright mafia will abuse it to snatch people for "crimes" which would be perfectly legal domestically. An example? UK student Richard O'Dwyer, who is facing extradition to the US for running a site which linked to torrent files - something perfectly legal in the UK, but illegal in the US:

Until last year, when police and US officials first visited him at his student accommodation in Sheffield, O'Dwyer ran a website called TVShack which provided links to other sites where users could download pirated versions of films and television shows. He appeared before magistrates in the capital this week for a preliminary hearing into the planned extradition, which he is fighting.

[...]

O'Dwyer's mother says she is baffled why a case with no direct links to the US – her son last went there aged five – should be heard in the US. Her lawyers agree.

"The (computer) server was not based in the US at all," O'Dwyer's barrister, Ben Cooper, who has also been heavily involved in the McKinnon case, told Tuesday's hearing at Westminster magistrates court. "Mr O'Dwyer did not have copyrighted material on his website; he simply provided a link. The essential contention is that the correct forum for this trial is in fact here in Britain, where he was at all times."

This isn't an accident; the US has admitted it is part of a global campaign to target copyright infringement, which asserts jurisdiction even when there is no link to the US:

British website owners could face extradition to the US on piracy charges even if their operation has no connection to America and does something which is most probably legal in the UK, the official leading US web anti-piracy efforts has told the Guardian.

The US's Immigration and Customs Enforcement agency (ICE) is targeting overseas websites it believes are breaking US copyrights whether or not their servers are based in America or there is another direct US link, said Erik Barnett, the agency's assistant deputy director.

This is simply abusive. But its what happens when your government is bought and paid for by a rapacious industry hellbent on protecting its obsolescent business model.

New Zealand has an extradition treaty with the US [PDF], which seems to have the same flaw as the US-UK agreement (no article on "forum"). But the good news is that it does not seem to cover copyright infringement or other intellectual property crimes. I think we need to keep an eye on our government, to make sure that it is not extended to include them in the future.