A friend of mine, a few years ago, had his stock options vest. He didn't sell the stocks. The stocks tanked a few months later. The IRS said he owed income tax on the value of the stocks when they vested.

He owed more tax than his net worth, lost his house, everything, and wound up in a trailer.

He never saw the money he was taxed on.

> bad faith arguments

A person's net worth can have wild gyrations on a daily basis. It's not unusual for a stock to move 10% in a few hours. MSFT dropped something like a third of its value last year. What something is "worth" is an utterly arbitrary notion, and basing taxes on that is inevitably unfair an inequitable. (A lot of effort and handwaving is done by accountants trying to guess at what something is "worth".) Heck, what is your house "worth"? Do you agree with the tax assessor? I once told the assessor that if he believed my house was worth what he assessed it at, I'd sell it to him at a 10% discount and he can flip it for what he thought it was worth. He wouldn't take the deal.

With taxes on income, that is fairly well understood and can be accounted for to the dollar.