"Our worthless currency keeps falling in value because we print it like toilet paper to pad our own pockets without pissing everyone off by openly raising taxes. You tried to avoid our hidden inflation tax by using our worthless currency to buy something we can't print. Since you did that, our currency has fallen in value even more. We're going to keep assuming you're an idiot and call the amount of value our currency lost against your purchase "capital gain", and demand you pay us a tax on that. So essentially we're going to shake you down for part of the value you didn't lose because you were smart enough not to hold the currency we keep inflating. Y U mad bro?" -the government