Because no one ever got outraged at fluffy old Karl Marx, dear me no. I agree with Plasmon that there are important differences between the two quotations other than what political tribe they come from, and that the words attributed to the Bush aide suggest a contempt for "judicious study" and looking before one leaps, which Marx's aphorism doesn't. But even if we set that aside and stipulate that the two quotations convey the exact same meaning and connotations, the point you seem to be making -- that the Bush guy got pilloried for being from the wrong tribe, whereas everyone loves Karl Marx when he says the same thing because he's from the right tribe -- seems to me badly wrong. First of all, if you think Marx is of the same political tribe as most people who take exception to the Bush aide's remarks, you might want to think again. That's a mistake of the same magnitude (and perhaps the same type?) as failing to distinguish Chinese people from Japanese because they all look "Oriental". Secondly, while Marx's aphorism gets quoted a lot, I don't think that's because everyone (or everyone on the "left", or whatever group you might have in mind) agrees with it. It expresses an interesting idea pithily, and that suffices.

The parent comment and the replies to it are a case study in how different people intuitively draw strongly opposed conclusions from a piece of ambiguous, politically charged evidence.