All three were headed to Canada and the latest two were already in Canadian airspace before being taken down by the US military. Reports don't make it seem like they were anything Canadian forces wouldn't be able to handle on their own, so why is the US military so concerned with patrolling Canadian skies? If you want to run with the given theory, perhaps it is that Canada is well aware of them and doesn't want them surveilling Canadian lands, thus the US military is taking them down at their expense to keep peaceful relations.