Also if you think about that - 1 million after taxes (and you probably need to pay taxes twice - first in the USA and then in Russia) is not that much money for someone who wants to demonstrate his principles.
And for some organizations having him would bring millions in grants, so if he declines the prize from these people - there is a reason for that. There was a great article in newyorker, in a few words, the world of science smells.
http://slashdot.org/story/06/08/25/2346220/New-Yorker-on-Per...
1. He dislikes what he perceives to be the corrupting effects of money and wants to guard against their influence in his life.
2. He thinks about pure math all day, and this makes him happy. Has solved for his basic needs. Does not want to worry about managing $1M. If and when he needs more money, he will deal w/ it then. Since he is a famous mathematician, it is reasonable to expect that others will help him out, if he asks.
These are variations on theme, actually. He enjoys thinking about math. He doesn't need $1M to think about math. Ergo, he doesn't need $1M.
The fallacy there is the assumption that he will never need $1M to think about math. He could get hit by a bus next week and need $1M for medical care.
(A quick Wikipedia check says that Russia has public health care supplemented with private providers. Suppose here that his injuries were bad enough that the public system wouldn't completely heal him.)
But the idea that he wouldn't have other resources to draw on, short of the $1M prize, seems wrong.
If you consider "economics" to deal only with maximizing money, it's economically non-rational to decline this prize. If you consider "economics" to deal with maximizing utility, it could easily be rational to decline this prize.
If he had a family to support, it might be a different matter.
Jason Fried and DHH talk extensively about the corrupting power of money. Needing to get paying customers in order to survive is a useful selective pressure. Jason Fried has a quote something like "Companies without funding get good at making money, companies with funding get good a spending it."
Both of these are economic reasons.
In addition, even if accepting the money would be a net gain for him, refusing it isn't the height of economic stupidity, as you suggest. Doing something that caused him to LOSE money is surely more stupid. Refusing the $1m is simply neutral. Not good, not bad. Medium stupid.
And lastly, likening him to people with Down Syndrome (and other disorders that fall under the umbrella of "retardation"), suggesting "well there's stupid, and then there's DOWN SYNDROME" is distasteful. If you think he's being epically stupid, that's fine. But likening him to an already maligned social class in order to make that point is not nice.