My guess is that Harper is doing this to look tough on the US, stealing that perennial election placard from the Liberals. The US will stand tough for a time, but will eventually cave against a strong onslaught from the Canadian government.
I'll bet Arar comes off the list 2-3 weeks before the next election call. A little thank you for Canada's assistance in Afghanistan.
Our participation in the asian country has made Canada a great friend to the United States, and provided Harper with an incredible amount of political capital down south.
In the end Arar will come off the list, Harper will get good pre-election press for picking a fight against the US and winning, and the US gets the help of an important ally in the struggle against the Taliban.
Getting results x3.