There is no reasonable public transportation from the areas of San Francisco where people live and Google's campus in Mountain View. The reality is that Caltrain mostly goes through residential areas, so any company that wants a public-transportation-friendly location is kind of out of luck without being located in the city. And of course, 4th and King is not very close to where most people in San Francisco live.
I've timed out how long it would take to commute from downtown San Francisco to Google's campus using only public transportation. It's on the order of 2 hours, and only if you go during rush hour and get the Caltrain shuttle and the Baby Bullet train. Outside of rush hour, it takes over 3 hours. Nobody is going to do a 6 hour daily commute; that's insane.
(I've also done the reverse commute -- living in Santa Clara and commuting to Google SFO. By gBus it only takes an hour and a half, since there is a bus from the apartment where I usually stay to the MTV campus, and then a bus from MTV to SFO. If you want to take public transportation, you're out of luck: it's a $20 cab ride to the Santa Clara train station, and then the usual Caltrain + Muni ride to the office.)
So the busses aren't a sign of privilege and elitism and class and exuberant riches and gentrification or whatever "bad" buzz words the leaders of Occupy Wall Street think they are. They're a way of cutting the emissions (and stress) of Googlers' commutes and a way of making practical something many people want: to live in the city but work in the suburbs.
It's true that the real fix would be to nuke the entire San Francisco Bay Area and rebuild it to be less car-centric, but that's not going to happen. (Especially in California, where they envy the timeliness of projects like the Second Avenue Subway and the Big Dig.) You can't undo 100 years of intentional discrimination against non-car-owners overnight. So there are busses instead: they use the infrastructure available frugally, save time, and lower the impact on the environment. If you hate that, it's possible that the problem is you, not some busses.