As an aside, the US seems like a very weird place to me. There are homeless people everywhere. I have never seen so many homeless people anywhere in my life...even in developing countries. It is crazy. I don't understand how people tolerate that. Not only poor people but the US seems to be a country which breeds a total lack of empathy, I don't understand how wealthy people tolerate it...and then you read the hyperbolic comment elsewhere on this thread about a founder saying this is theft, this well end Silicon Valley, it is insanity, it is intolerable...try tolerating sleeping in a tent under a bridge.
Either way, the reason you tax the $100m is so you don't end up with a govt that will take more than your money. The notion that you can just take forever has never been a part of politics.
As an aside, I think most Americans perception of authoritarian govts is that they are anti-democratic in this way...but if you look at monarchies, there was always responsibility from the wealthy, there was always a safety valve...the US has no safety valve, there is no social responsibility from the wealthy, they have taken everything and are asking where the rest is (and sound utterly deranged when you suggest they should make $5m rather than $10m). There are things much worse than taxation, you have to live in the world with other people.
EDIT: I will add...I would kind of quibble with the notion that markets are free in the US. The amount of state support for capitalists since 2008 (not talking about TARP which was necessary) has been very significant. Free money for the wealthy, usurious interest and rent for the poor. In particular, the level of downward social mobility in the US is very low for a capitalist economy. Markets are free in the US...in one sense.