I believe Nikon lenses (Nikkor) are covered by worldwide warranty regardless of where purchased.
However, bodies are more local, and warranties vary on where you buy it from.
As an example, I bought my D5100 from the US, and it has a five year (extended) warranty in the US (because I registered it in 30 days).
However, I live in the UK, and Nikon Europe (which covers UK), will only 'warranty' it for one year from date of purchase regardless of what the US warranty was/is.
My lenses however were covered under a two year US warranty, and Nikon Europe cover them the same as lenses have a worldwide warranty
Maybe they have this distinction as lenses are more universal - you don't need to package them with say a US charger as opposed to a UK charger or a EU charger etc