They zero out the low fare buckets so that pax won’t book the flight so they don’t have too many to rebook later when they cancel; they do this to other flights in other markets as well (markets which don’t require any DOT approvals) when they know they will eventually cancel the flight.

There is no need to use such crude methods just to show the DOT there is no demand; they can easily show the reduced number of pax who are flying a PDX-TYO itinerary currently using one-stop connecting itineraries (since no airline has a PDX-TYO nonstop anyway).