In reducing the number of shares on the market by buying back existing stock, it offers stability for shareholders who will continue to hold stock in the company by increasing the value of the shares left on the market.
Businessweek also noted Hiroshi Yamauchi's heirs, who inherited almost 10 percent of company stock following his death last year, sold back some of their stake in the company. Given the high inheritance tax they have to pay, it's possible that they're covering that cost under the buyback terms. Nintendo does not know exactly how many shares the heirs sold back.
Not that it matters. Unless they somehow bogarted the entire buyback, which would've sucked for other shareholders trying to cash out.