Part of me suspects he could have stayed had his manifesto not gotten picked up on the front page of CNN. There was only one clear way for the Google brand to distance itself then. The message is clearly think what you want, but if your words end up next to the Google logo on national news...

Edit: thinking about that, he couldn't have gone far in the company though without some serious apologies or something, because I can't see a manager assigning him to work in or lead a team with women on it, and that's 1/3rd the company. Maybe it's also a practical decision to fire him and be done with it even if it didn't leak.