He doesn't 'give money away' no-string-attached. He uses the donations to control the way the economy deals with the externalities of capitalism. It's fundamentally anti-democratic for that share of the world's economy to be driven by the whims of one man.
Not saying everyone needs to have the same level of say in the outcome of the economy, but there's a minimum below which the system becomes unresponsive to real human need and a maximum above which diminishing returns sets in.
