For the sake of offering a counter argument
1. Just a calrification Jorge and Casey don't want to race 75km from the nuclear reactor. They have stated that they would be willing to race elsewhere in Japan.
2. VR and Dani have both publicly stated they dont want to go either. Dani has said he will not race there either. The difference is Casey and Jorge have been the most vocal.
3. The entire motoGp grid minus Ayoma signed a petition stating they don't want to go there. The only difference is that Casey and Lorenzo are not going to lose their job due to there talent, where any one outside of alien status would lose theirs. So they can speak out on behalf of the entire grid.
4. The day Dorna released their report this came out link
5. "Their objections are based not so much on the radiation at the Motegi circuit, but more on the situation at the stricken Fukushima Daiichi nuclear power plant. The Fukushima prefecture continues to be rocked by aftershocks, some as high as magnitude 6.4, and there is a broad concern inside the paddock that the situation at the plant is not under control, and that a large earthquake during the MotoGP weekend could cause the plant to suffer further damage and lead to a large-scale release of radiation." link
6. The fact of the matter is that Dorna want to go there so badly due to contractual obligations not due to raising Japanese moral. There are still 5000 people that are unaccounted for in the earth quake stricken part of Japan. A MotoGP race is not going to make their families "feel" any better.
7. As for the economic influence of a MotoGP race in Japan, in the immediate area it will be a surplus but as for the country it will not even make a dent. for reference please check the stock index of Toyota, Sony, Honda or any large Japanese company prior to the earth quake until now. The dip and upswing you will see in stock values have nothing to do with MotoGP.
This is just a counter argument, I know people tend to get heated on here when counter points are put out there. So don't take any of the above personal.