It's all down to what they think they can get away with charging - we are, after all, known as "rip-off Britain".
Some years ago I was in Vermont and bought a Cannondale mountain bike. After paying import duty and VAT it still cost me less than half the extortionate amount they charge for Cannondales here.
The worst though, was when in Boston I came across a Church's shoe shop which was selling shoes (made in Northampton UK) for far less dollars than we pay pounds. So it's not all about freight and taxes.