FWIW, I have never seen them offered at face. Makes sense to me. These 500's are different size/weight/metal from the circulation coins, so vending machines won't take them and Japan loves its vending machines.
The Japanese market for coins seems to be very thin and to a large extent coin collectors also collect stamps. The typical Japanese stamp collector will do modern commemorative stamps only, as the older definitives are very expensive. They may have carried over their habits to coins.
It's a shame we don't have any Japanese members...
Peter