The reason the superwealthy are often so liberal is often a question of scale; if you make a billion dollars a year, whether the tax rate is 30% or 40% is irrelevant. You will have to make do with a $600 million net rather than a $700 million disposable income. How many shirts can you wear at one time? How many 30,000 square foot mansions can you sleep in?
The effect of the estate tax is strongly anti family, even if it only attacks relatively wealthy families. People who run the family business, whether it is a farm, a scrap yard, or an investment pool often see the capital asset not as their personal asset but as an asset that must be preserved for following generations much as it was preserved for them. Wealth building is not always an act that occurs in a single generation.
A high estate tax causes the wealthy to either spend the money while they are alive rather than give it to the government, causing even more ostentatious and conspicuous consumption. It causes money to be spent of complicated trusts and insurance policies rather than more productive assets. It penalizes illiquidity and risk taking, causing productive assets to be liquidated to pay taxes.
The American economy no longer exists in a vacuum. High estate taxes will encourage the wealthy to seek to live in a country without a predatory stance on their wealth. John Templeton became a Bahama citizen so that he could give his wealth to Christian charities rather than the U.S. government. (Yes, many of the wealthy want to give the money away, but want a say in who gets it.) To try and restrict this capital flow, as one Georgia Congressman suggested, would possibly destroy our dominance as the safest haven for capital in the world.
If an American makes a billion dollars on a stock portfolio he is supposed to give 50% to the government when he dies according to Warren Buffet. Yet if a Saudi prince (or some other foreign national) makes a billion dollars on the same portfolio he keeps it all. Does this seem right to Mr. Buffet? Capital will go where it is respected.
Warren Buffet owns insurance companies who benefit from estate planning to avoid taxes. He buys successful business that often face the liquidation decision to generate liquidity to pay estate taxes. Perhaps his stance in favor of estate taxes is self serving, perhaps not. It is not in the best interest of the economy or the families who have spent generations creating wealth.
Class warefare is no more appealing when proposed by the wealthiest among us.