Because all else being equal if you can avoid an exposure of 10's of msv, you do, just like if you are getting over a bad cold you avoid driving if you can because you might get lightheaded and run off the road, but you almost certainly won't. And because in the very epicenter of a crisis, you can't predict exactly what's going to happen next and so you act conservatively.
The funny thing about your logic is, it's how the people in charge of Chernobyl thought too. They couldn't simply work out the likely exposures and take action accordingly --- at least, not at first. So they spent zillions of dollars on cleanup and mitigation but didn't tell farmers not to pass unchecked milk on to children, who subsequently contracted thyroid cancer because of the uptake of radioactive iodine. You are illustrating the point. "If they evacuate", you think, "there must be some gigantic risk they aren't telling us about".
Oh, now I get it - radiation does not kill people. Anti nuclear power people kill people.
Your example shows one aspect of why people dislike nuclear power: it's easy to see how to avoid getting into a car accident, but apparently less easy to see how to react in case of a nuclear disaster. Normal people wouldn't know that they should check the milk.
Also I don't want to live in a world where I have to check everything for radiation before I consume it.
As for your other question: so now I am a troll, just because I disagree with you? Or because the comments are indented more than x levels? And yes, I have several issues with this discussion. Not the least I dislike emotional argumentation, and I think in this case it is the pro nuclear power people who are actually emotional: they feel that the other side is emotional, and react emotionally to that. I also dislike mindless herds "oh it is XKCD, so it must be cool. I want to be on the side of XKCD" (not that I accuse you of this, but if you remember, I merely took issue with the XKCD chart).
What I get from HN is that nuclear power is also a thing for elitist people, who look down on the stupid people who can't understand physics well enough.
I hope you're not a troll. But asking things like "so why don't we evacuate a 50km radius around cigarettes" is trollish. And you should read more. This stuff isn't that complicated. Watch the video at the top of the thread (you clearly haven't, or you wouldn't be talking about plutonium exposure in Japan).
The funny thing about your logic is, it's how the people in charge of Chernobyl thought too. They couldn't simply work out the likely exposures and take action accordingly --- at least, not at first. So they spent zillions of dollars on cleanup and mitigation but didn't tell farmers not to pass unchecked milk on to children, who subsequently contracted thyroid cancer because of the uptake of radioactive iodine. You are illustrating the point. "If they evacuate", you think, "there must be some gigantic risk they aren't telling us about".