Suggestion for equal pie??? - Every sentence that comes from Wayne Swan's lips to start with
To your incentive question - it's quite simple...... If the rate of taxation is too high then people will find other avenues to invest / divert their money that ultimately give them a higher rate of return / less taxation expense.
Plus - it reduces the incentive to work harder/more (aka invest more of your own, at your own risk, to generate more wealth).
Your final sentence may have been in jest - but there is a LOT of that mentality out there.
Hell - even for average (or less than average wage earners). How many times have you heard people say "Oh I don't want to work the extra day cause it will push me over and they'll take more tax out of my pay!". There are a lot of non-rich that come from that starting position (even if it's a flawed view of how taxation works). Perception is reality.
At the top end - they understand reality more than most.