I definitely don't buy it either.
The market has nothing to do with morality. It's simply a question of efficiency: are people as a whole better off, utility wise? But as long as everyone is making voluntary, non-coerced trades (labor for $, $ for products), then by definition they are better off. There's no moral aspect to it, just economic efficiency/"progress".
But that doesn't mean you get to keep everything you make. The market is a tool, not an end. If you kept everything, there'd be no taxation, no government, no education, nothing else, and it would be a moral disaster. The moral element comes into play with taxation: because you (as a billionaire) benefit extraordinarily from the country's systems put into place -- contract enforcement, courts, roads, education of your workers, everything -- you are morally obligated to pay your "fair share" of that back.
The only people who claim people are entitled to 100% of the money they make are a subset of libertarians/anarchists. It's not a very popular opinion, since it makes society pretty much unworkable.