I'm surprised that people are asking how / why he is giving money away and no one mentioned his Giving Pledge: https://en.wikipedia.org/wiki/The_Giving_Pledge Bill Gates and Warren Buffet pledged to give half of their net worth away during their life or death to charity. They're practicing what they preach.
it's capitalism at is best you start a company in your garage take it to be worth Bilions an then give it all away, doesn't get better than this.
"Capitalism at it's best" isn't people giving away money... it's people being competitively compensated for their contributions to society-- the ability to take risks and reap the rewards.
Of course then you can argue that tax is a way to enforce a minimal amount of philanthropy onto people, forcing them to contribute to the greater good whether they want to or not... but people who seem to be very vocally in favour of pure capitalism also seem to be very vocally against high taxes.