The Buy American program applies to US stimulus spending. That is money the US government is spending to stimulate the US economy. Why would they use it to stimulate other economies?
To put this in context, imagine not that the US stimulus was being spent on projects for Canadian firms... imagine it was being spent on Chinese imports.
Stimulus of the US economy will pull up everyone else, and Canada in particular, in any event.
At the least, I don't think this issue is as black and white as many are making out.