"Why the federal estate tax is unfair and should be eliminated: Most of those earnings have been taxed already, so why should those who have prospered not be able to keep them?"
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Simple question.....that can be answered rather simply: Because "those who have prospered" are not "the ones who keep it." 
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Bill Gates dies and leaves me with a cool $1 Billion. What did I do to earn that money? Nothing. Bill Gates earned it, not me. So is it "fair" that I pay Estate Taxes on the Windfall that I get for money that I did not work to earn? 
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That's the question. Is it fair to impose an Estate Tax on the Windfall that children might get when their parents die.....when they are not the ones who worked or invested to build up that Windfall in the first place?
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But the reverse is also true: Is it fair for The Government to impose an Estate Tax and reap More Money and steal/take money away from children when their wealthy parents die? 
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Someone gets Money. Why should it be The IRS?