For him, marginal dollars have negative utility. This may be because of morality -- he could consider the act of giving mathematics prizes antithetical to promo

For him, marginal dollars have negative utility. This may be because of morality -- he could consider the act of giving mathematics prizes antithetical to promoting good math research; it may be because of his focus -- he knows that he would have to manage the money if he got it, which would distract him from doing math; it may be because of fear -- there may be people that would kidnap his family to get access to that money. 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.