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Google is an international company. It has customers in many countries that practice many religions, or none at all. Just a glance at Wikipedia shows that, in addition to Christians, there are 1.5 billion Muslims, 1.1 billion non-religious people, and 1 billion Hindus, in the world. If Google starts observing religious holidays in anything more than a secular manner (Santa at Christmas, for example), there are going to be people upset that either 1)Google is promoting a religion other than theirs, or 2) Google is not promoting their religion sufficiently.
Keep in mind that Easter is not celebrated on the same day by all sects, so an Easter Doodle on the date Orthodox celebrate Easter might well result in angry reactions from Catholic followers. Cesar Chavez is a legitimate historical figure that helped improve the lives of many people. Choosing him for the Doodle avoids controversy except from people that feel private companies should acknowledge and honor their chosen religious beliefs (or, to be fair, perhaps feel that all religious faiths should be equally honored, and would appreciate it if Google also promotes evens such as Eid, Ramadan, Diwali, Rosh Hashanah, and the Winter Solstice, in addition to those of their own faith).