Except that it is. Japan is self-sufficient for food. Land marked for growing rice is essentially "eternally locked" to growing rice; you cannot sell it or use it for other purposes, and are required to plant it.
There are a lot of subsidies for Japanese rice growers too but I think this is primarily because without them they would not be competitive with American imports.
Also, the issue of US rice exports was one of the biggest stumbling blocks for US-Japanese TPP negotiations, so I don't think Japanese rice producers believe Japanese people will not eat medium-grain rice.