You do know that Page and Brin are Jewish, right?
This has already been stated. But today is the holiest day of the year for an estimated 2 billion of the planet's total human population. If you're of a different religion, or are not religious, and you run the world's most visited website, you use your traditional logo. You don't go off celebrating the life of someone who has little to do with Christianity. Granted it's their website and they can do what they want with it, but it just doesn't make good business sense. And they knew this. They just decided to put Cesar Chavez on their site because they knew it would generate hits, tick people off, and get them talking.