It's an interesting argument, and fungibility is a critically-important feature of Bitcoin. However, I don't think this is at all an unexpected move by the IRS

It's an interesting argument, and fungibility is a critically-important feature of Bitcoin. However, I don't think this is at all an unexpected move by the IRS. Most of us were expecting this all along, and calculating our taxes accordingly. Now, if the U.S. were smart and forward-looking we'd adopt the same policy as Denmark and Germany (Bitcoins aren't taxed). But we're not, and so we didn't.