When you have a vulnerability that is not easily or quickly addressed, you should not call undue attention to it. Such a tit-for-tat could escalate in other ways. Does the US want to continue to use the Seaway? Or maybe Canada needs to conduct lengthy inspections of vessels carrying American cargo before they can pass through those waterways.
From a domestic security standpoint, there is an argument to be made for not relying (solely) on pipelines that pass through the US. However, if the US were escalating to the point of cutting off our energy supply, what's to say they wouldn't use kinetic means to cut our pipelines and critical infrastructure. There is next to nothing we could do to stop them, such a strike could be done in hours. Where such an attack would go is unclear, occupying Canada would be untenable. If they couldn't hold Iraq or Afghanistan, I don't think they could hold Canada.
