3kW isn't a magic number for the kettle, it's just the amount of power you can get away with drawing when you've got a 240V supply rated at 13A. The more power, the faster the water boils.
As I understand it, freestanding kettles are much less popular in North America, presumably because a hob can draw more power?
This extends to home espresso makers. Some international models of home espresso makers get rave reviews in AU/NZ, but lackluster reviews in the US. Only so many watts you can get on 120V before you exceed amperage constraints.
In case you're not familiar with them, we're discussing something like [1], or, to show these are very widespread commodity appliances, [2], which costs $7 including tax.
[1] http://www.amazon.co.uk/Andrew-James-Cordless-Indicator-Warr...
[2] http://www.argos.co.uk/static/Product/partNumber/9016710.htm