Compare the seatpost QR to the handlepost extension QR, and you will see a variation on mechanical advantage. While the slot is about the same length allowing a 'squeeze zone', the seatpost QR is placed at the extreme top affording maximum 'crush'. Oddly, the handlepost QR is centered in the slot, requiring more force to exert the same hold.

I'll bet that moving the screw hole / QR position up about a quarter inch would go a long way towards clamping it sufficiently to prevent unwanted motion.