First of all, I searched and couldn't find he's actually giving $10b. What I mean is that I don't know if the $10b accounts for any Amazon investments / costs.
Second, it's not -$10b if he's getting a tax break for some of this. It's $7.6b as someone else mentioned. I know, still a lot of money! But he's not loosing $10b from his fortune.
Third, in this case you happen to think it's a win for society and the future of humanity. Other billionaires give money for causes they believe, like The Koch family for "advancing liberty and freedom", or Bloomberg advocating for a third term as mayor. The lack of accountability for the influence billionaires have in our society should be a concern. We praise when they give to causes we believe, but the system that allows them to do this has nefarious byproducts. It's like we're watching billionaires shape our society and our culture and think that's alright.