While Google surely puts more thought into how its practices affect society than the average giant corporation, it is hardly a charity and I don’t think this statement should be any indication that it plans on becoming one. "Don't be evil" is still a long way on the altruism scale from becoming a humanitarian NGO.
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Is it, though, appropriate for a head of a public company to officially align said company on a piece of controversial (and not strictly relevant to the business) legislation? Well, perhaps not. In this situation however, I think it was an admirable and fitting action. It reinforces Google’s "not business as usual" stance and commitment to social awareness and doing good. Of course, though, there will be many who see this as the opposite of "doing good."
The values of tolerance, diversity, and meritocratic inclusion are, though, so fundamental to the company (and, when at its best, the Valley and the tech industry in general) that this seems like a natural and reasonable move. Though I agree there is certainly room for argument, particularly over specifics, here, many will surely see opposition to Prop. 8 as reflective of the above values. I took Sergey’s blog post as more a statement of "live and let live" and respect for all than as a focused and specific endorsement of gay marriage.
I don't work at Google but I do live in the Bay Area and I'm confident that the employee culture at the company is such that those with strong feelings against gay rights are used to keeping their position largely to themselves. In other words, conservative Christian/Jewish/Muslim/Sikh/etc. individuals who work there have, I’m sure, resigned to keep their religious convictions a personal matter in order to effectively and civilly participate in a diverse environment. Consequently, I would be surprised if this pronouncement causes much of a stir even from pro-Prop. 8 Googlers.