There is generally a lot of availability for Japanese domestic flights and they are very cheap at 6000 miles and no tax. But with UA you have to do it by phone. Another quirk is that ANA (and JAL) release domestic flights only 60 days in advance. I guess it's different for NRT connectors. Nobody goes to NRT for a domestic flight (well, I guess if you live in parts of Chiba or Ibaraki) so they are all for connections to intl flights.