A forced air system circulates air within the house, but generally doesn't do anything to bring in "fresh" air to replace the "stale" air. Maybe since the house is older, the poor insulation/air sealing meant that the bathroom fan was indeed enough to pull air in through all the various cracks? I can't really imagine having an "open" vent in the basement in Alberta though, temperatures get way too cold for that to make any sense. There are some electronically controlled ones, but still, you'd be dealing with major condensation issues.
It surely sounds like it was done intentionally though.