Either way, if tower first notified Air Canada of a serious incident, then they too had a duty to preserve the CVR. It's not only the pilots.
I'm speculating that -- at the time -- neither the pilots, Air Canada, nor the controller had an appreciation of just how close they were to disaster, until the tapes were reviewed.
The pilots probably thought they'd just need to file a safety report in the morning and that would be the end of it.