The Smoking Gun says the IRS won't comment on how Bofill, who moved back to his native Spain after an unsuccessful stint as a day trader, managed to earn the $500 million or so in 2006 that would justify such a tax bill. But a commenter there has a plausible theory!
He probably didn't file a tax return. They have the records of all his daytrading sales. When you don't file a tax return, the IRS assumes that each of your sales is 100% profit. So he did not have to earn $500 million, he only had to buy and sell $500 million worth of stock. This actually isn't as crazy as it sounds. $2.5 million dollars in trades per day isn't all that much for someone who might make 10 to 100 trades a day.
There you have it: there is not one god damn good thing about day trading.