What are the biological processes of ingesting radioactive particles?
Your question has a rather glaring assumption which is that the amount of radiation emitted by food sources is equivalent to the amount of radiation received while sitting on top of the Fukushima plant. Even if a fish was as radioactive as an equal mass of concrete at the site, the dose you are getting at the site is because of all of the other things surrounding you.

The answer is simply that the food sources near the plant have been deemed (by the officials you mention) to emit a dose of radiation that is less than the level deemed harmful (again, by the officials you mention). It is of course possible that either the estimate of the radiation dose from the food is low, or the level deemed harmful is too high; that discussion is too broad for an answer here.