Donating part of the proceeds of an item for sale is as much about driving sales as it is about the particular company being solely motivated by good will. I refer you to the article linked below. As an added bonus for Blizzard, who spent next to nothing creating these virtual pixels, because the donation will be made in their name and not yours they will get a significant tax write off over and above the millions of dollars they make themselves.
Compare and contrast the Blizzard offer to Yoko Ono's donation of
ALL the proceeds from iTune sales of John Lennon's Give Peace a chance to the United Nations until the end of the year if you want to "measure" the scale of Blizzard's generosity.
If Blizzard were truly motivated by Charity they would voluntarily donate to a variety of charities and/or Champion a particular cause and create a program to promote giving to charities using their vast customer base similar to Ronald McDonald houses which are sponsored by the company as well as individual donations at franchises or any other countless examples.
While this "offer" is without a doubt a good deal for the Children's Wish Foundation if you think Blizzard's main motivation is charitable giving I have some swamp land in Florida to sell you.
http://abcnews.go.com/Business/story?id=4022433&page=1