The Ministry of Agriculture, Forestry and Fisheries notified the food industry on April 20 that the industry should abide by the radiation safety standards set by the national government, and not use their own (i.e. lower) standards.
The reason? Actually there are two:
So that the testing is "scientific" (meaning if companies are going to test on their own they are strongly encouraged to do so at the testing laboratories registered with the Ministry - there are 40 of them in Japan according to the Ministry's information);
To avoid "excessive" regulation and to use the government standards in presenting the results.
From Asahi Shinbun (4/21/2012):
Stop using radiation standards in food that is different from the national standards, Ministry of Agriculture notifies
The Ministry of Agriculture, Forestry and Fisheries (MAFF) issued the notice to the 270 industry groups such as supermarkets, food manufacturers and restaurants on April 20 that they should abide by the safety standards for radioactive materials set by the national government. The purpose is to restrain the move by the food industries to set their own, stricter standards and conduct testing, and to sell only the food not contaminated with radioactive materials.
The national government adopted the new safety standards for food in April (100 becquerels/kg of radioactive cesium in food in general, 50 becquerels/kg for milk and baby food, and 10 becquerels/kg for drinking water).
The notice was signed by the head of the Food Industry Bureau of MAFF, and written to warn on voluntary inspection of radiation in the private sector. The notice emphasizes that the new government standards are even stricter than the international indicators, and says "To prevent the excessive regulation and confusion at the time of food consumption, we would like you to make it well known that the results of voluntary inspections should be interpreted according to the standards set by the Food Sanitation Act.
Let's all get internal radiation exposure, the government seems to be saying. After all, this government doesn't think anything of feeding the imperial family with milk with radioactive cesium. Why should it care for lowly citizens? The DPJ government also plans to send Prince and Princess Akishino, parents of the future emperor (5-year-old Prince Hisahito), to Uganda, of all places.