Allowable intake levels are different for different isotopes. Potassium has a biological half life of a few hours, Caesium (the isotope in question here), of several weeks.
The FDA apparently doesn't even bother with regulating allowable Potassium-40 levels due to this.
Which is why you shouldn't use Bq/kg and instead use Sieverts. It's not that bad, human body can get rid of caesium-131 etc. pretty well via kidneys. The main problem was radioiodide which accumulates in thyroid and parathyroid glands and super heavy isotopes which are chemically dangerous, e.g. plutionium and uranium - but these do not disperse far at all.
https://en.wikipedia.org/wiki/Sievert - Recommended reading, has a handy table. Fukushima doesn't even register as dangerous in there, equivalent to about 3 full CT scans.
Data from Chernobyl is scarce, but I'd suspect 4x that value due to more retained isotopes.
The US FDA thinks 1000 Bq/kg for Cs-137 is just fine.[1] Browsing around, I have seen other sources that go higher. I am not knowledgeable enough to know if one or both of those limits is overly conservative. Considering my experience with other food safety limits, I would say they probably are.
German and EU authorities.
> What are the actual levels? How do they compare to the radioactivity of other foods?
http://www.sueddeutsche.de/bayern/hohe-radioaktive-werte-wil...
Legal levels are <400 Bq/kg for baby food and 600 Bq/kg for other foods; contamination in these cases ranges from 1000 to 10000 Bq/kg.