Very clever strategy to only ban some, but not all Japanese prefectures.
Firstly, this reduces the supply drop and thus price pressure for Chinese consumers.
Secondly, this promotes conflict within Japan between those affected and those not affected, whereas if China blanket banned all Japanese seafood, it will ironically create a United front in Japan, since everyone there will be in the same boat.
Lastly and most importantly, this will create the market conditions to ensure Japanese people themselves end up eating the overwhelming majority of contaminated seafood since exporters will buy up as much of the seafood from non-banned areas as economically viable, while seafood from banned areas will plummet in price until eventually someone (local) is willing to eat it.