Why?
Regarding the Qantas flight, supposedly a international carrier can't operate a domestic only ticket in the USA, so hence they can't sell a LAX-JFK ticket. But if a passenger was to fly SYD-LAX on Monday they can then take the LAX-JFK leg on Qantas on Thursday since they are on a intl ticket.
So maybe similar here. LH can't operate a domestic only ticket within Japan.