It's ok to do it with government because we're supposed to all be equal under the law.
That's ironic to me, because that's pretty much the perspective I'm coming from. The question of wealth redistribution - especially the issue of doing it via progressive income tax - seems to be about treating people quite unequally. You have to do some pretty convoluted gymnastics with the concept of equal (introducing the very subjective notion of 'equal burden') to claim that progressive taxation represents equality under the law.