I don't think greed is a bad (or an avoidable) thing. If you design a system where you say to people "vote like this and you will earn one dollar, but vote like that and you will earn two dollars" them voting the second way isn't surprising, and should not be judged negatively.
A system that requires, in order to function properly, participants to ignore their own immediate financial best interests is not a viable system.
A system that can be ruined by greed never had a chance in the first place.
True. But this argument doesn't prove linear is bad, nor prove it's good.
IMHO, a little greed is good, but too greed is not good, so it's good to give some financial incentives but not too many. Over-rewarding is often counter-productive.