The fact that you can redeem your miles early is precisely why it is not a pyramid scheme. In a pyramid scheme, the late comers must wait in que for those higher up the pyramid to be paid off.
That is correct and therefore pyramid schemes are illegal. Airlines modified the system so we might have to call it "modified, legal pyramid".
My point is to show that airlines restrict US (MR's) because we use weird routings to get enough miles for redemption. I don't feel guilty for that. My free ticket is paid a thousand times by people who never have enough miles to redeem.