I think you don't realise the true cost... In the U.S. they have a single continental sized country to supply with Internet content, almost all of that coming from that same country.
In New Zealand almost all content comes from overseas.
While there are 120 million Internet users in the U.S, there are only 3 million Internet users in New Zealand (approximate numbers).
It's obivious the cost is very different. There's no way it would be economically viable to offer unlimited traffic here. By the way, even in the U.S. the ISPs use Fair Use Policy and will cut people that use too much.
I suppose it would be costly for Telecom to provide unlimited traffic...And NZ is a relatively small country so you wouldn't expect what you would get in other economically developed countries such as USA or Japan. I hope Telecom will release ADSL 2+ soon, a small but great leap in technology.