The issue resolves itself when you are both geographically in the country and admitted by immigration. When travelling from YYZ to the U.S., both countries for immigration purposes consider me in the U.S.A. once I've gone through pre-clearance. If I change my mind or the flight is cancelled I have to go through Canadian immigration to be readmitted. For non-immigration purposes however I am still in Canada so should I commit a crime on the airside I will be charged and prosecuted by Canadian authorities.
Pre-clearance is very old, but the requirement to re-enter Canada formally is relatively recent in comparison. Years ago, you just exited back out through the US inspection area, there was no reason to re-clear Canada immigration. Note that pre-clearance is a convenience service, it does not bestow any territorial or legal authority to the US government aside from processing the entrance to the USA of yourself and goods accompanying you. In fact, while in the pre-clearance area you are still in Canada and subject to Canadian, not US law (except for Customs and Agricultural violations inside the pre-clearance area). If the US authorities wanted to arrest you for an outstanding warrant or other violation, they have no authority to do so, they need to call the airport police to do it, and you would still be subject to the standard extradition treaty process where the request could be denied.
