Whatever you think of the memo, you have to be a little concerned about the precedent this would set if companies were not allowed to discriminate based on 'political' views. Should I be forced to hire a neo-nazi or a terrorist sympathiser so long as they're up to the job?
Companies can simply have a straight-edged policy of no politics -- from religion to economics, it's all off-topic and out-of-bounds.
It is illegal to prevent people from engaging in politics, in California.