No your not. You're pre-cleared to enter the US. You are still in Canada and that part of the terminal does not constitute US territory in the same way a consulate or embassy does. The employees working in the terminal after all don't need to clear US Customs. Theres a reason US Customs stamps your boarding pass, and thats to let the airport agents know you've been cleared...

Edit to add: The US Customs facilities clearly state that you are still on Canadian territory, but that you are inspected by US officers for clearance to enter the US (or something to that effect). US Customs have no powers of arrest or detention, and thats why you always see Peel Police standing by, as they are the ones that have to make an arrest on behalf of US Customs if theres a reason to do so. I had an argument about this point with my Politics TA a few weeks back who claimed to have seen US Customs carrying guns (which I've never seen and doubt is true), she over-dramatized her experience at Customs in order to claim there was a security hysteria after 9/11 (which arguably is true, but her example was hardly a good one).