I can understand why flights to Japan would canceled right now. For one thing, the whole country is in a state of emergency right now. The last thing they need is more people, especially outsiders, getting in the way. I think the primary reason is that the airports are extremely busy right now with the incoming aide from other countries. Rescue teams are flying in even from China. Food and medical supplies are being sent in. Nearly all of it is coming by air which is huge amount of air traffic. Also, Japan's newest airport protrudes into the ocean and was badly damaged by the tsunami. It's probably not usable yet.
If panic burns calories, some people here seem to be trying to start a weight loss club. Right now, CSNBC and the many other broadcasters are practically making radiation "experts" out of people who use microwave ovens. They are blowing the radiation risk out of proportion in order to scare people and increase their ratings. The fact is that we are exposed to all sorts of ionizing radiation in our daily lives. The soil contains radioactive materials and we are bombard with cosmic radiation from space. Every time you inhale dust, you are taking small amounts cesium and other radioactive materials into your body. The whole planet was exposed to radioactive fallout from cold war nuclear testing and it is still in the soil. We have always had some radiation in our lives.
The people who have been exposed to radioactive contaminates were working at the nuclear plants and none of them show levels like those at Chernobyl. After decontamination, the likelihood that any of them will come to harm by it is extremely small. When there is a reason to worry your government will tell you. Iodine tablets are very common and plentiful. There are people buying up local stocks of them and then spreading panic so they can resell them at a premium to people who don't have much sense. This happens whenever there is a big disaster.
Right now, the rest of the world is shook up by the scale of the disaster in Japan. When things calm down, these people will realize that they were never in any danger. We need to focus on helping the victims not acting out.