They don't care so much about the Bitcoins themselves as they do getting their cut of the fiat money when you cash them out, I'd wager. If you buy 300 bushels

They don't care so much about the Bitcoins themselves as they do getting their cut of the fiat money when you cash them out, I'd wager. If you buy 300 bushels of apples for $1 and, for whatever reason, the price spikes to $300 overnight, when you sell them do you claim that "Well, I paid the tax on the $1 so I don't have to pay taxes on the $299"? If you keep your money in apples the IRS doesn't want 20% of your apples. It's only if you sell them back out and claim it as income.