I was travelling with a Canadian passport. I arrived at San Francisco, with a subsequent connecting flight to Toronto. The agent asked me the usual questions, I showed him my ticket etc... He then asked me to look at the camera and place my fingers on the scanner. This was in 2005, and had I had never seen this before and he said that all foreigners travelling in the US had to do this. Why was it that even though I was a Canadian on a short visit (transiting), was forced to get finger printed even before this?

They can search anything that comes into the U.S. through post.