"It is local politics that is keeping them offline, just as it is politics rather than evidence that keeps the exclusion zone around Fukishima so large."
No, it's radioactive caesium contamination, combined with democratically accountable representatives making hard choices, advised by unelected public servants who know what the internationally accepted safe limits are and don't want to exceed these either for genuine concern over victims health, and/or due to concern about future litigation by anyone who goes back and gets cancer for whatever reason and sues for negligence based on provable violation of international safety standards.
http://en.wikipedia.org/wiki/Radiation_effects_from_Fukushima_Daiichi_nuclear_disaster states: 'As of February 2012, the crippled Fukushima nuclear plant is still leaking radiation and areas surrounding it could remain uninhabitable for decades due to high radiation. It could take “more than 20 years before residents could safely return to areas with current radiation readings of 200 millisieverts (mSv) per year, and a decade for areas at 100 millisieverts per year” '
50mSv /year is the maximum allowed for US nuclear workers: http://en.wikipedia.org/wiki/Orders_of_magnitude_%28radiation%29
http://www.iaea.org/Publications/Factsheets/English/radlife.html#dose states: "On average, our radiation exposure due to all natural sources amounts to about 2.4 mSv a year" and this same article (tag harmful) states: "With all the knowledge so far collected on effects of radiation, there is still no definite conclusion as to whether exposure due to natural background carries a health risk, even though it has been demonstrated for exposure at a level a few times higher."